Implement short circuit for base change only

Implement short circuit for base change only

Allow mode to be set with fb_id set to -1, meaning set
the mode with the current fb (if we have one bound).

Allow intelfb to hook back up it's fb if modesetting
clears it (maybe temporary).

Move any crtc->fb related register changes to set_base
in intel_fb.

General intelfb cleanups.