drm/mgag200: Remove P_ARRAY_SIZE

Replace P_ARRAY_SIZE by array pre-initializing and ARRAY_SIZE(). No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-4-tzimmermann@suse.de