drm/vc4: Add some missing HVS register definitions.

At least the RGBA expand field we should have been setting, because we
aren't expanding correctly for 565 -> 8888.  Other registers are ones
that may be interesting for various projects that have been discussed.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stefan Schake <stschake@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523479755-20812-2-git-send-email-stschake@gmail.com