modesetting: Disable pageflipping when using a swcursor

The miPointerSpriteFunc swcursor code expects there to only be a single
framebuffer and when the cursor moves it will undo the damage of the
previous draw, potentially overwriting what ever is there in a new
framebuffer installed after a flip.

This leads to all kind of artifacts, so we need to disable pageflipping
when a swcursor is used.

The code for this has shamelessly been copied from the xf86-video-amdgpu
code.

Fixes: xorg/xserver#828


Signed-off-by: Hans de Goede <hdegoede@redhat.com>