drm/vc4: tests: Drop drm parameter for vc4_find_crtc_for_encoder

The DRM device pointer and the DRM encoder pointer are redundant, since
the latter is attached to the former and we can just follow the
drm_encoder->dev pointer.

Let's remove the drm_device pointer argument.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>