Restore our saved values for backlight. This way even with fastset on
S4 resume we will correctly restore the backlight to the active values.

Changes since v1:
- Call enable_backlight() when backlight.level is set. On suspend
  backlight.enabled is always cleared, this makes it not a good
  indicator. Also check for crtc->state->active.
Changes since v2:
- Use the new update_pipe() callback to run this on resume as well.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Tolga Cakir <cevelnet@gmail.com>
Cc: Basil Eric Rabi <ericbasil.rabi@gmail.com>
Cc: Hans de Goede <jwrdegoede@fedoraproject.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190108160842.13396-1-maarten.lankhorst@linux.intel.com