drm/amdgpu: add vmhub funcs helper (v2)

This patch is to introduce vmhub funcs helper to add following callback
(print_l2_protection_fault_status). Each GC/MMHUB register specific programming
should be in gfxhub/mmhub level.

v2: remove the condition of funcs assignment.

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>