Set the default to 600Mhz if it's not set in the bios,
and bump the default to 600Mhz if it's lower than that.

This fixes display issues with certain 4k DP monitors when
using 5.4 Ghz DP clocks.

v2: fix typo.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>