drm: Remove two leaks of vblank reference count in error paths.

If the failing paths were hit, the vblank IRQ would never get turned off
again.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>