drm/i915/skl: Update watermarks before the crtc is disabled.

On skylake some of the registers are only writable when the correct
power wells are enabled. Because of this watermarks have to be updated
before the crtc turns off, or you get unclaimed register read and write
warnings.

This patch needs to be modified slightly to apply to -fixes.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92181
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: stable@vger.kernel.org
Cc: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447945645-32005-4-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>