drm/i915/opregion: abstract acpi display type getter for a connector

No functional changes.

Reviewed-and-tested-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/ef19172e5a00d8abd8190a5389283ef6b5f7eaa9.1465810007.git.jani.nikula@intel.com