drm/radeon/kms: don't use 0 bpc for adjusting hdmi clock

If the bpc is set from the connector is 0, we then use it later to adjust
in a special case the HDMI pixel clock, however if the bpc is 0, we end up
passing a 0 pixel clock into the code.

I'm not sure if this is the correct answer or if we should avoid the HDMI
clock adjustment for 0 values.

This fixes a divide by 0 on my Llano system with a HDMI monitor and hdmi
audio enabled.

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