drm/amdgpu: Use better names to reflect it is CP MQD buffer

Add "CP" to AMDGPU_GEM_CREATE_MQD_GFX9 to indicate it is only for CP MQD
buffer.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>