drm/amdkfd: Only keep release_mem function for Hawaii

release_mem is only used for Hawaii, but because GFX7 and GFX8 share the
same function pointer structure, so we only delete release_mem for GFX9
and GFX10.

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