drm/vkms: move wb's atomic_check from encoder to connector

As the renamed drm_atomic_helper_check_wb_connector_state() now accepts
drm_writeback_connector as the first argument (instead of drm_encoder),
move the VKMS writeback atomic_check from drm_encoder_helper_funcs to
drm_connector_helper_funcs. Also drop the vkms_wb_encoder_helper_funcs,
which have become empty now.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231208010314.3395904-3-dmitry.baryshkov@linaro.org