drm/vc4: Fix potential null pointer read when disabling vblank

vc4_disable_vblank assumed that vc4_encoder was always assigned,
which isn't guaranteed.

If it isn't assigned then disable the interrupt anyway as it's
not connected.

https://github.com/raspberrypi/linux/issues/6146



Fixes: 63c0bcc4 ("drm/vc4: Add option to call from crtc to encoder on vblank")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>