drm/radeon/kms: fix DVO setup on some r4xx chips

Some r4xx chips have the wrong frev in the
DVOEncoderControl table.  It should always be 1
on r4xx.  Fixes modesetting on DVO on r4xx chips
with the bad frev.

Reported by twied on #radeon.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>