drm/bridge: Add RGB to VGA bridge support

Some boards have an entirely passive RGB to VGA bridge, based on DACs
implemented by resistor ladders.

Those might or might not have an i2c bus routed to the VGA connector in
order to access the screen EDIDs.

Add a bridge that doesn't do anything but expose the modes available on the
screen, either based on the EDIDs if available, or based on the XGA
standards.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160930143709.1388-3-maxime.ripard@free-electrons.com

drivers/gpu/drm/bridge/Kconfig

@@ -17,6 +17,13 @@ config DRM_ANALOGIX_ANX78XX
 	  the HDMI output of an application processor to MyDP
 	  or DisplayPort.
 
+config DRM_DUMB_VGA_DAC
+	tristate "Dumb VGA DAC Bridge support"
+	depends on OF
+	select DRM_KMS_HELPER
+	help
+	  Support for RGB to VGA DAC based bridges
+
 config DRM_DW_HDMI
 	tristate
 	select DRM_KMS_HELPER