drm/amdgpu: rename AMDGPU_GFXHUB/MMHUB macro with hub number

The number of GFXHUB/MMHUB may be expanded in later ASICs.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>