drm/radeon: change cac_weights_* to static

Sparse reports these issues
si_dpm.c:332:26: warning: symbol 'cac_weights_pitcairn' was not declared. Should it be static?
si_dpm.c:1088:26: warning: symbol 'cac_weights_oland' was not declared. Should it be static?

Both of these variables are only used in si_dpm.c.  Single file variables
should be static, so change their storage-class specifiers to static.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>