If the crtc is active, we can simply flip a new fb onto it, provided the
other mode setting reqs are met.  Otherwise, we'll need to do a full
mode set to re-enable the crtc.

v2: check for crtc active and set mode_changed accordingly
v3: add module parameter, i915.fastboot, to control no fb -> fb flip behavior
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>