drm/Kconfig: add missing 'depends on DRM' for DRM_DP_CEC

Add a missing 'depends on DRM' for the DRM_DP_CEC config
option. Without that enabling DRM_DP_CEC will force CEC_CORE
to =y instead of =m if DRM=m as well.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/489bdaae-9b05-2d70-12e1-4fda7899dfc1@xs4all.nl

drivers/gpu/drm/Kconfig

@@ -168,6 +168,7 @@ config DRM_LOAD_EDID_FIRMWARE
 
 config DRM_DP_CEC
 	bool "Enable DisplayPort CEC-Tunneling-over-AUX HDMI support"
+	depends on DRM
 	select CEC_CORE
 	help
 	  Choose this option if you want to enable HDMI CEC support for