drm/i915: Disable crtcs gracefully before GPU reset on gen3/4

The GPU reset also resets the display on gen3/4. The g33 docs say we
should disable all planes before flipping the reset switch. Just
disable all the crtcs instead. That seems a nicer thing to do anyway.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>