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