amdgpu/dc: Fix double unlock in amdgpu_dm_commit_planes

Reported by smartch:
amdgpu_dm_commit_planes() error: double unlock 'spin_lock:&crtc->dev->event_lock'
amdgpu_dm_commit_planes() error: double unlock 'irqsave:flags'

The error path doesn't return so we only need a single unlock.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>