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.

Port of radeon commit:
9368931d

v2: clean up the code a bit

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=91896Signed-off-by: Alex Deucher <alexander.deucher@amd.com>