drm/i915: write backlight harder

drm/i915: write backlight harder

02ae6fd77749087cf17f4b18e5a4b7183a6d26da is the first bad commit
commit 02ae6fd77749087cf17f4b18e5a4b7183a6d26da
Author: Takashi Iwai <tiwai@suse.de>
Date:   Sat Aug 11 08:56:42 2012 +0200

    drm/i915: Fix blank panel at reopening lid

changed the register write sequence for restoring the backlight, which
helped prevent non-working backlights on some machines. Turns out that
the original sequence was the right thing to do for a different set of
machines. Worse, setting the backlight level _after_ enabling it seems
to reset it somehow. So we need to make that one conditional upon the
backlight having been reset to zero, and add the old one back.

Cargo-culting at it's best, but it seems to work.

Cc: stable@vger.kernel.org
Cc: Takashi Iwai <tiwai@suse.de>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=47941
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>