drm/radeon: properly filter DP1.2 4k modes on non-DP1.2 hw

The check was already in place in the dp mode_valid check, but
radeon_dp_get_dp_link_clock() never returned the high clock
mode_valid was checking for because that function clipped the
clock based on the hw capabilities.  Add an explicit check
in the mode_valid function.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=87172



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