drm/i915: Allocate state checker crtc state

Don't clobber the old state with the results of the hardware readout.

In particualr this clobbers old_crtc_state->commit which caused
explosions once I hooked up vblank works. Not sure why this doesn't
cause problems already?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>