drm/amdgpu: stop/resume fb access when gpu reset V3

V2: Add wait_for_mc_idle after stopping fb access
V3:
	1. Remove resume fb access since there is no need to
   	do that for gpu reset.
	2. Move stop fb access to amdgpu_gpu_reset function,
	since it's the same for all asics.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com> (V1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>