modesetting: Disable pageflipping when using a software cursor

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: #828 (closed)

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