drm/vc4: Add missing NULL check to vc4_crtc_consume_event

vc4_crtc_consume_event wasn't checking crtc->state->event was
set before dereferencing it, leading to an OOPS.

Fixes "a5b534bb drm/vc4: Resolve the vblank warnings on mode switching"

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>