virgl: Don't support QUADS natively

Using quads leads to a rather expensive buffer readback when quads are stored in display lists, so avoid them altogether.

Closes: #5825 (closed)

Compared to the alternative to use write-only fences this seems to be the better alternative: On virgl (AMD Radeon 6700 XT) I have for glxgears:

• no patch 5 fps
• add write-only fences to not wait for buffer reads 800 fps
• this patch 5500 fps