radeon: add AGPMode 1 quirk for RV250

The Intel 82855PM host bridge / Mobility FireGL 9000 RV250 combination
in an (outdated) ThinkPad T41 needs AGPMode 1 for suspend/resume (under
KMS, that is). So add a quirk for it.

(Change R250 to RV250 in comment for preceding quirk too.)

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org