drm/i915: Revive combination mode for backlight control

drm/i915: Revive combination mode for backlight control

This reverts commit d580bdc79df7737f09cfed1ce27e424dad9cadcf

    drm/i915: Do not handle backlight combination mode specially

since this commit introduced other regressions due to untouched LBPC
register, e.g. the backlight dimmed after resume.

In addition to the revert, this patch includes a fix for the original
issue (weird backlight levels) by removing the wrong bit shift for
computing the current backlight level.
Also, including typo fixes (lpbc -> lbpc).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34524
Acked-by: Indan Zupancic <indan@nul.nu>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>