drm/i915: enable/disable backlight for eDP

Like we already do for the LVDS panels. This seems to help greatly
in setting up the backlight, since the BIOS might refuse to cooperate.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
v2: Move the backlight_off call from panel_off to edp_backlight_off,
noticed by Paulo Zanoni.

Reviewed-by: Paulo Zanoni <przanoni@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>