drm/vc4: use drm_hdmi_connector_mode_valid()

Use new drm_hdmi_connector_mode_valid() helper instead of a
module-specific copy.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241130-hdmi-mode-valid-v5-6-742644ec3b1f@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>