drm: Add an interface to reset the device

Iterate over the attached CRTCs, encoders and connectors and call the
supplied reset vfunc in order to reset any cached state back to unknown.
Useful after an invalidation event such as a GPU reset or resuming.

Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>