drm/i915: move "no VBT in opregion" quirk to intel_opregion_setup()

Check the quirk in intel_opregion_setup(), and don't initialize
opregion->vbt at all if the quirk says it's not present, hiding the
quirk from the rest of the driver.

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/7cdc86eb441f8b7075142445a800b07ecf8c76cb.1450089383.git.jani.nikula@intel.com