drm/amdgpu: move get_invalidate_req function into gfxhub/mmhub level

This patch is to move get_invalidate_req into gfxhub/mmhub level. It will avoid
mismatch of the different gfxhub/mmhub register offsets and fields in the same
gmc block.

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>