drm/i915: Check for platform specific GPIO config

Panel GPIO control should be done based on platform. Add a check
to restrict VLV and CHT specific GPIO confirguration, so that
they dont apply to other platforms.

The VBT spec fails to mention the PMIC backlight control option is valid
only for VLV/CHT, and the field may be set to "PMIC" for BXT even if
PMIC is not desired or possible.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
[Jani: amended commit message a bit and fixed indentation.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486551058-22596-2-git-send-email-vidya.srinivas@intel.com