drm/i915/backlight: setup and cache pwm alternate increment value

This will also be needed later on when setting up the alternate
increment in backlight enable.

Cc: Shawn Lee <shawn.c.lee@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/9984b20bc59aee90b83caf59ce91f3fb122c9627.1474281249.git.jani.nikula@intel.com