drm/vc4: Make sure that vblank waits work without v3d loaded.

This flag exists to protect legacy drivers, but when vc4's v3d doesn't
probe, it doesn't get set up by vc4_v3d.c's call of drm_irq_install.
This resulted in applications running as fast as possible, and laggy
performance from compton as it had to wait for the latest rendering by
the application for its presentation.

Signed-off-by: Eric Anholt <eric@anholt.net>