drm/i915: Parse panel backlight controller from VBT

Currently the backlight controller is taken as 0. It needs to derive
value from the VBT. Adding the necessary changes.

v2 by Jani:
 - drop obsolete comments, drop redundant initialization (Bob)
 - merge debug logging into one

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Tested-by: Bob Paauwe <bob.j.paauwe@intel.com>
Tested-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481189178-426-1-git-send-email-jani.nikula@intel.com