drm/radeon/kms: allow max clock of 340 Mhz on hdmi 1.3+

hdmi 1.3 raises the max clock from 165 Mhz to 340 Mhz.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>