drm/amdgpu: Add xcc specific functions for gfxhub

GFXHUB 1.2 supports multiple XCC instances. Add XCC specific functions
to handle XCC instances separately.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>