drm/amd/pm: unified lock protections in amdgpu_dpm.c

As the only entry point, it's now safe and reasonable to
enforce the lock protections in amdgpu_dpm.c. And with
this, we can drop other internal used power locks.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>