drm/i915: introduce macros to check pipe config properties

This code will get _really_ repetive, and we'll end up with tons more
of this kind. So extract the common patterns.

This should also help when we add a lazy pipe_config compare mode for
fastboot.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>