drm/i915/backlight: setup backlight pwm alternate increment on backlight enable

Backlight enable is supposed to do a full setup of the backlight. We
were missing the PWM alternate increment bit in the south chicken
registers on lpt+ pch. This potentially caused a PWM frequency change
when the chicken register value was lost e.g. on suspend.

v2 by Jani, rebase on the patch caching alt increment

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97486
References: https://bugs.freedesktop.org/show_bug.cgi?id=67454


Cc: Cooper Chiou <cooper.chiou@intel.com>
Cc: Wei Shun Chen <wei.shun.chang@intel.com>
Cc: Gary C Wang <gary.c.wang@intel.com>
Cc: stable@vger.kernel.org # v4.4+ 32b421e7 drm/i915/backlight: setup and cache...
Cc: stable@vger.kernel.org # v4.4+
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Shawn Lee <shawn.c.lee@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/8265f5935bd31c039ddfc82819d26c2ca1ae9cba.1474281249.git.jani.nikula@intel.com