drm/amdgpu/amdkfd: clean up mmhub and gfxhub includes

Use the appropriate mmhub and gfxhub headers rather than adding
them to the gmc9 header.

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