drm/mgag200: Enable MGA mode during device register initialization

MGA cards can run in traditional VGA mode or an enhanced MGA mode; with
the latter being required for KMS. So far, MGA mode was enabled during
modesetting. As it's fundamental for device operation, the patch moves
it next to the device register setup.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730102844.10995-5-tzimmermann@suse.de