drm/vc4: Mark the device as active when enabling runtime PM.

Failing to do so meant that we got a resume() callback on first use of
the device, so we would leak the bin BO that we allocated during
probe.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 553c942f ("drm/vc4: Allow using more than 256MB of CMA memory.")