drm: Fix locking in drm_atomic_helper_resume

In the conversion to drop drm_modeset_lock_all and the magic implicit
context I failed to realize that _resume starts out with a pile of
state copies, but not with the locks. And hence drm_atomic_commit
won't grab these for us.

v2: Add locking checks in helpers to make sure we catch this in the
future. Note we can only require the locks in the atomic_check phase,
not in the commit phase. But since any commit is guaranteed to first
run the checks (even for the resume stuff where we use stored
duplicated old state) this should give us full coverage. Requested by
Maarten.

Cc: Jyri Sarha <jsarha@ti.com>
Fixes: a5b8444e ("drm/atomic-helper: remove modeset_lock_all from helper_resume")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170531083813.1390-1-daniel.vetter@ffwll.ch