drm/radeon: fix DP mode validation

Switch the order of the loops to walk the rates on the top
so we exhaust all DP 1.1 rate/lane combinations before trying
DP 1.2 rate/lane combos.

This avoids selecting rates that are supported by the monitor,
but not the connector leading to valid modes getting rejected.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=95206



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org