drm/radeon/kms: remove useless clock code

This code was originally for forcing some clocks on certain asics.
However, this code was later moved to asic specific functions
for all of the affected asics.  The only users of the original
code at this point were r600, rv770, and evergreen and the code
was not relevant for those asics.  So, remove it.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>