drm/i915/dsi: Group MIPI_SEQ_BACKLIGHT_ON/OFF with panel_[en|dis]able_backlight

Execute the MIPI_SEQ_BACKLIGHT_ON/OFF VBT sequences at the same time as
we call intel_panel_enable_backlight() / intel_panel_disable_backlight().

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1488374106-4949-7-git-send-email-jani.nikula@intel.com