drm/amdgpu: use function pointer for gfxhub functions

gfxhub functions are now called from function pointers,
instead of from asic-specific functions.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>