drm/amdgpu: move struct gart_funcs into amdgpu_gmc.h

And rename it to struct gmc_funcs.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Samuel Li <Samuel.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>