drm/amdgpu: Add AMDGPU_GPU_PAGES_IN_CPU_PAGE define

To hopefully make the code dealing with GPU vs CPU pages a little
clearer.

Suggested-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>