drm/amdgpu: fix checkpatch.pl warning to amdgpu_drv.c

fix checkpatch.pl WARNING:
Prefer 'unsigned int' to bare use of 'unsigned'

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>