drm/i915: Enable VBT based BL control for DP

drm/i915: Enable VBT based BL control for DP

Currently, BXT_PP is hardcoded with value '0'.
It practically disabled eDP backlight on MRB (BXT) platform.

This patch will tell which BXT_PP registers (there are two set of
PP_CONTROL in the spec) to be used as defined in VBT (Video Bios Timing
table) and this will enabled eDP backlight controller on MRB (BXT)
platform.

v2:
 - Remove unnecessary information in commit message.
 - Assign vbt.backlight.controller to a backlight_controller variable and
   return the variable value.
v3:
 - Rebased to latest code base.
 - updated commit title.

Signed-off-by: Mustamin B Mustaffa <mustamin.b.mustaffa@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227030734.37901-1-mustamin.b.mustaffa@intel.com