drm/panel-simple: Allow the bus format to be read from DT for panel-dpi

The "panel-dpi" compatible string configures panel from device tree,
but it doesn't provide any way of configuring the bus format (colour
representation), nor does it populate it.

Add a DT parameter "bus-format" that allows the MEDIA_BUS_FMT_xxx value
to be specified from device tree.

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