drm/amdgpu: rename smumgr to smum for dpm

Rename smumgr.h to smum.h, because smum.h is to align with the dpm of
other chips and we will use "iceland_smumgr" at powerplay in following
patches.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>