runqemu: unbreak non-gl displays

Correct two issues:
1. Looking for dri.pc is only needed when gl is enabled.
2. virtio-vga-gl works only when gl is enabled via -display,
otherwise virtio-vga needs to be selected.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>