drm/amdgpu/ttm: optimize vram access in amdgpu_ttm_access_memory()

1. using vram aper to access vram if possible
2. avoid MM_INDEX/MM_DATA is not working when mmio protect feature is
enabled.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>