drm/bridge: dw-hdmi: Make DRM_DW_HDMI selectable

Commit c0e0f139 ("drm: Make drivers depends on DRM_DW_HDMI") turned
select dependencies into depends on ones. However, DRM_DW_HDMI was not
manually selectable which resulted in no way to enable the drivers that
were now depending on it.

Fixes: 4fc8cb47 ("drm/display: Move HDMI helpers into display-helper module")
Reported-by: Mark Brown <broonie@kernel.org>
Reported-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240403-fix-dw-hdmi-kconfig-v1-2-afbc4a835c38@kernel.org

drivers/gpu/drm/bridge/synopsys/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config DRM_DW_HDMI
-	tristate
+	tristate "Synopsys Designware HDMI TX Controller"
 	depends on DRM_DISPLAY_HDMI_HELPER
 	depends on DRM_DISPLAY_HELPER
 	select DRM_KMS_HELPER