drm/i915: Protect cxsr state with wm_mutex

Let's protect the cxsr state with the wm_mutex, since it might
get poked from multiple places if there's a parallel plane update
happening with a pipe getting enable/disabled.

It's still pretty racy for the old platforms, but for vlv/chv it
should work, I think. If not, we'll improve it later anyway.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-10-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>