drm/amd/pm: fix race in power state check before mutex lock

drm/amd/pm: fix race in power state check before mutex lock

The power state check in amdgpu_dpm_set_powergating_by_smu() is done
before acquiring the pm mutex, leading to a race condition where:
1. Thread A checks state and thinks no change is needed
2. Thread B acquires mutex and modifies the state
3. Thread A returns without updating state, causing inconsistency

Fix this by moving the mutex lock before the power state check,
ensuring atomicity of the state check and modification.

Fixes: 6ee27ee27ba8 ("drm/amd/pm: avoid duplicate powergate/ungate setting")
Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>