v6 of the structure was programmed incorrectly:

  args.v6.ulCrtcPclkFreq.ulPixelClock = cpu_to_le32(clock / 10);

ulPixelClock is a 24-bit bitfield. This statement would thus
do a 32-bit swap of (clock / 10) and drop the top 8 bits which
are ... the LSB. Not what we want. Instead use masks & shifts.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>