drm/radeon: add properties to configure the width of the underscan borders

This allows for a more exact fitting on the physical
display. The new properties default to zero which corresponds to the
previous underscan border width[height] formula:
(display_width[display_width] >> 5) + 16.

Example to set a horizontal border width of 30 and a vertikal border
height of 22:

   xrandr --output HDMI-0 --set underscan on --set "underscan hborder" 30 --set "underscan vborder" 22

Signed-off-by: Marius Gröger <marius.groeger@googlemail.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>