drm/i915/dsi: run backlight on/off sequences in panel enable/disable hooks

Based on the documentation alone, it's anyone's guess when exactly we
should be running these sequences. Add them where it feels logical. The
drm panel hooks don't currently offer us more granularity anyway.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/267c4a2bde2076af18e9b8335c0bef2e26ea3112.1474286487.git.jani.nikula@intel.com