drm/radeon/kms/atom: get better min pixel clock info

Where supported use ulMinPixelClockPLL_Output rather than
usMinPixelClockPLL_Output for pll_out_min.  This seems to
improve pll selection on some boards.

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