drm/amdgpu: Sprinkle drm_modeset_lock_all to appease locking checks

In

commit 7a3f3d66


Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jul 9 23:44:28 2015 +0200

    drm: Check locking in drm_for_each_connector

I added locking checks to drm_for_each_connector but failed that
through drm_helper_connector_dpms -> drm_helper_choose_encoder_dpms
it's used in a few more places in the amdgpu resume/suspend code.

Fix them up.

Note that we could use the connector iterator macros in there too, but
that's for the future.

Port of radeon commit:
drm/radeon: Sprinkle drm_modeset_lock_all to appease locking checks

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>