drm/i915/gvt: Add new debugfs tool mmio_diff

drm/i915/gvt: Add new debugfs tool mmio_diff

This new debugfs entry is used to figure out which registers of vGPU
is different to host. It is a useful tool for new platform enabling
and debugging. When read this entry, all the diff mmio are recognized
and sorted by mmio offset. Besides, the bit positions of different
value are listed in 'Diff' column. Here is a show:

$ sudo cat ./mmio_diff
Offset   HW       vGPU     Diff
00002030 000025f8 00000000 3-8,10,13
00002034 012025f8 00000000 3-8,10,13,21,24
00002038 027fb000 00000000 12-13,15-22,25
0000203c 00003000 00000000 12-13
00002054 0000000a 00000040 1,3,6
00002074 012025f8 00000000 3-8,10,13,21,24
00002080 fffe6000 00000000 13-14,17-31
000020a8 fffffeff ffffffff 8
000020d4 00000004 00000000 2
....
00145974 eb42718c 010c11b0 2-5,13-14,17-19,22,25,27,29-31
00145978 0000002f 0000002a 0,2
0014597c 0000002f 0000002a 0,2
00145980 0000002b 00000028 0-1
00145984 a5a87c9e b27d20c0 1-4,6,10-12,14,16,18,20,22-26,28
001459c0 88390000 883c0000 16,18
00146200 88350000 883a0000 16-19
Total: 72432, Diff: 901

v3: fix a typo.
v2: add mmio_hw_access_pre/post().

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>