drm/amd/pm: drop redundant MEM_TYPE_* macros

As these are already defined in amdgpu_atombios.h. Otherwise, we may
hit "redefined" compile warning.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>