drm/i915/vlv: T12 eDP panel timing enforcement during reboot

The panel power sequencer on vlv doesn't appear to accept changes to its
T12 power down duration during warm reboots. This change forces a delay
for warm reboots to the T12 panel timing as defined in the VBT table for
the connected panel.

Ver2: removed redundant pr_crit(), commented magic value for pp_div_reg

Ver3: moved SYS_RESTART check earlier, new name for pp_div.

Ver4: Minor issue changes

Ver5: Move registration of reboot notifier to edp_connector_init,
      Added warning comment to handler about lack of PM notification.

Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>