drm/i915: Parsing the PWM cntrl and CABC ON/OFF fields in VBT

For dual link panel scenarios there are new fields added in the
VBT which indicate on which port the PWM cntrl and CABC ON/OFF
commands needs to be sent.

v2: Moving the comment to intel_dsi.h(Jani)

v3: Renaming the field names (Jani)

v4 by Jani: make this patch only about VBT

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Yetunde Adebisi <yetundex.adebisi@intel.com>
Signed-off-by: Deepak M <m.deepak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459346623-30752-2-git-send-email-jani.nikula@intel.com