drm/amdgpu: no need to clean debugfs at amdgpu

drm/amdgpu: no need to clean debugfs at amdgpu

drm_minor_unregister will invoke drm_debugfs_cleanup
to clean all the child node under primary minor node.
We don't need to invoke amdgpu_debugfs_fini and
amdgpu_debugfs_regs_cleanup to clean agian.
Otherwise, it will raise the NULL pointer like below.
[   45.046029] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a8
[   45.047256] PGD 0 P4D 0
[   45.047713] Oops: 0002 [#1] SMP PTI
[   45.048198] CPU: 0 PID: 2796 Comm: modprobe Tainted: G        W  OE     4.18.0-15-generic #16~18.04.1-Ubuntu
[   45.049538] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[   45.050651] RIP: 0010:down_write+0x1f/0x40
[   45.051194] Code: 90 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 55 48 89 e5 53 48 89 fb e8 ce d9 ff ff 48 ba 01 00 00 00 ff ff ff ff 48 89 d8 <f0> 48 0f c1 10 85 d2 74 05 e8 53 1c ff ff 65 48 8b 04 25 00 5c 01
[   45.053702] RSP: 0018:ffffad8f4133fd40 EFLAGS: 00010246
[   45.054384] RAX: 00000000000000a8 RBX: 00000000000000a8 RCX: ffffa011327dd814
[   45.055349] RDX: ffffffff00000001 RSI: 0000000000000001 RDI: 00000000000000a8
[   45.056346] RBP: ffffad8f4133fd48 R08: 0000000000000000 R09: ffffffffc0690a00
[   45.057326] R10: ffffad8f4133fd58 R11: 0000000000000001 R12: ffffa0113cff0300
[   45.058266] R13: ffffa0113c0a0000 R14: ffffffffc0c02a10 R15: ffffa0113e5c7860
[   45.059221] FS:  00007f60d46f9540(0000) GS:ffffa0113fc00000(0000) knlGS:0000000000000000
[   45.060809] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   45.061826] CR2: 00000000000000a8 CR3: 0000000136250004 CR4: 00000000003606f0
[   45.062913] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   45.064404] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   45.065897] Call Trace:
[   45.066426]  debugfs_remove+0x36/0xa0
[   45.067131]  amdgpu_debugfs_ring_fini+0x15/0x20 [amdgpu]
[   45.068019]  amdgpu_debugfs_fini+0x2c/0x50 [amdgpu]
[   45.068756]  amdgpu_pci_remove+0x49/0x70 [amdgpu]
[   45.069439]  pci_device_remove+0x3e/0xc0
[   45.070037]  device_release_driver_internal+0x18a/0x260
[   45.070842]  driver_detach+0x3f/0x80
[   45.071325]  bus_remove_driver+0x59/0xd0
[   45.071850]  driver_unregister+0x2c/0x40
[   45.072377]  pci_unregister_driver+0x22/0xa0
[   45.073043]  amdgpu_exit+0x15/0x57c [amdgpu]
[   45.073683]  __x64_sys_delete_module+0x146/0x280
[   45.074369]  do_syscall_64+0x5a/0x120
[   45.074916]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

v2: remove all debugfs cleanup/fini code at amdgpu
v3: squash in unused variable removal

Signed-off-by: Yintian Tao <yttao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>