drm/amd/display: make some dc_dmub_srv functions static

Not used outside of dc_dmub_srv.c.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>