drm/mgag200: Call mgag200_device_probe_vram() from per-model init

Call mgag200_device_probe_vram() from each model's initializer. The
G200EW3 uses a special helper with additional instructions.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601112522.5774-5-tzimmermann@suse.de