drm/vc4: dpi: Add option for inverting pixel clock and output enable

DRM provides flags for inverting pixel clock and output enable
signals, but these were not mapped to the relevant registers.

Add those mappings.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20220613144800.326124-10-maxime@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>