drm/radeon/kms: set a default max_pixel_clock

On some Power rv100 cards, we have no ATY OF table, but we have
no combios table either, and hence we refuse all modes on VGA-0
since we end up with a 0 max pixel clock.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Cc: stable@kernel.org
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>