drm/i915: write backlight harder

drm/i915: write backlight harder

c425b53cba892f2c2b31371f75513204070a8e37 is the first bad commit
commit c425b53cba892f2c2b31371f75513204070a8e37
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>