drm/amdgpu: Remove struct drm_driver.gem_prime_mmap

drm/amdgpu: Remove struct drm_driver.gem_prime_mmap

The callback struct drm_driver.gem_prime_mmap as been removed in
commit 0adec22702d4 ("drm: Remove struct drm_driver.gem_prime_mmap").
Do not assign to it. The assigned function, drm_gem_prime_mmap(), is
now the default for the operation, so there is no change in functionality.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 0adec22702d4 ("drm: Remove struct drm_driver.gem_prime_mmap")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230619141129.2002-1-tzimmermann@suse.de