drm/vc4: Add support for per plane scaling filter selection

Seeing as the HVS can be configured with regard the scaling filter,
and DRM now supports selecting scaling filters at a per CRTC or
per plane level, we can implement it.

Default remains as the Mitchell/Netravali filter, but nearest
neighbour is now also implemented.

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