drm: meson: drv: use macro when initializing vpu

This patch add new macro which is used to set WRARB/RDARB mode of
the VPU.

Signed-off-by: Julien Masson <jmasson@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/86v9wv82f1.fsf@baylibre.com