drm/amdgpu: abstract set_vm_fault_masks function to refine the programming

This patch is to add set_vm_fault_masks helper to amdgpu_gmc to refine the
original programming.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>