Fix documentation build errors for amdgpu: correct the filename.

Error: Cannot open file ../drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
Error: Cannot open file ../drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
Error: Cannot open file ../drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c

WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -sphinx-version 5.3.0 -function MMU Notifier ../drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c' failed with return code 1
WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -sphinx-version 5.3.0 -internal ../drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c' failed with return code 2

Fixes: d9483ecd ("drm/amdgpu: rename the files for HMM handling")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>