drm: vc4: Probe DPI/DSI timings from the firmware

For DPI and DSI displays query the firmware as to the configuration
and add it as the only mode for DRM.

In theory we can add plumbing for setting the DPI/DSI mode from
KMS, but this is not being added at present as the support frameworks
aren't present in the firmware.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>