drm/edid: fix typo in 1600x1200@75 mode

Spotted by Scott Bertilson.
Fixes fdo bug 28146.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Mark Marshall <mark.marshall@csr.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>