drm/i915/edp: Get the Panel Power Off timestamp after panel is off

Kernel stores the time in jiffies at which the eDP panel is turned
off. This should be obtained after the panel is off (after the
wait_panel_off). When we next attempt to turn the panel on, we use the
difference between the timestamp at which we want to turn the panel on
and timestamp at which panel was turned off to ensure that this is equal
to panel power cycle delay and if not we wait for the remaining
time. Not waiting for the panel power cycle delay can cause the panel to
not turn on giving rise to AUX timeouts for the attempted AUX
transactions.

v2:
* Separate lines for bugzilla (Jani Nikula)
* Suggested by tag (Daniel Vetter)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101518
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101144


Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507135706-17147-1-git-send-email-manasi.d.navare@intel.com