drm/amd/display: dc/clk_mgr: make function static

[Why]
linux-next kernel test robot reported the following problem:
warning: no previous prototype for 'dce60_get_dp_ref_freq_khz' [-Wmissing-prototypes]

[How]
mark dce60_get_dp_ref_freq_khz() as static

Fixes: 3ecb3b79 "drm/amd/display: dc/clk_mgr: add support for SI parts (v2)"
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>