xfree86/modes: Bail from xf86RotateRedisplay if pScreen->root is NULL

Avoids a crash in xf86RotatePrepare -> DamageRegister during
CreateScreenResources if rotation or another transform is configured for
any connected RandR output in xorg.conf. The generic rotation/transform
code generally can't work without the root window currently.

Closes: xorg/xserver#969
Fixes: 094f42cd "xfree86/modes: Call xf86RotateRedisplay from
                     xf86CrtcRotate"
Acked-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>