drm/atomic-helper: rename drm_atomic_helper_check_wb_encoder_state

The drm_atomic_helper_check_wb_encoder_state() function doesn't use
encoder for anything other than getting the drm_device instance. The
function's description talks about checking the writeback connector
state, not the encoder state. Moreover, there is no such thing as an
encoder state, encoders generally do not have a state on their own.

Rename the function to drm_atomic_helper_check_wb_connector_state()
and change arguments to drm_writeback_connector and drm_atomic_state.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231208010314.3395904-2-dmitry.baryshkov@linaro.org