drm/dp/mst: Calculate MST PBN with 31.32 fixed point

commit a9ebb3e4 upstream.

Our PBN value overflows the 20 bits integer part of the 20.12
fixed point. We need to use 31.32 fixed point to avoid this.

This happens with display clocks larger than 293122 (at 24 bpp),
which we see with the Sharp (and similar) 4k tiled displays.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>