drm/mgag200: Warn once if trying to set start address on broken hardware.

Some MGA200 hardware is broken and can't use a start address > 0.

v2: Warn if startaddr is not 0, and hw doesn't support it.
  (instead of removing MGAG200_FLAG_HW_BUG_NO_STARTADD)
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504134026.283417-4-jfalempe@redhat.com

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -223,6 +223,9 @@ static void mgag200_set_startadd(struct mga_device *mdev,
 
 	startadd = offset / 8;
 
+	if (startadd > 0)
+		drm_WARN_ON_ONCE(dev, mdev->flags & MGAG200_FLAG_HW_BUG_NO_STARTADD);
+
 	/*
 	 * Can't store addresses any higher than that, but we also
 	 * don't have more than 16 MiB of memory, so it should be fine.