drm/atomic: Cleanup on error properly in the atomic ioctl.

It's probably allowed to leave old_fb set to garbage when unlocking,
but to prevent undefined behavior unset it just in case.

Also crtc_state->event could be NULL on memory allocation failure,
in which case event_space is increased for no reason.

Note: Contains some general simplification of the cleanup code too.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Add note about the other changes in here. And fix long line
while at it.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>