drm/amd/pm: fix compile warning about missing prototype for function

Fix the warning below:
>> drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c:1234:29: warning: no previous prototype for function 'asic_internal_ss_get_ss_table' [-Wmissing-prototypes]
   ATOM_ASIC_INTERNAL_SS_INFO *asic_internal_ss_get_ss_table(void *device)
                               ^
   drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomctrl.c:1234:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
   ATOM_ASIC_INTERNAL_SS_INFO *asic_internal_ss_get_ss_table(void *device)
   ^
   static
   1 warning generated.
Signed-off-by: Evan Quan <evan.quan@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/pm/powerplay/hwmgr/ppatomctrl.c

@@ -1231,7 +1231,7 @@ uint32_t atomctrl_get_mpll_reference_clock(struct pp_hwmgr *hwmgr)
 /**
  * Get the asic internal spread spectrum table
  */
-ATOM_ASIC_INTERNAL_SS_INFO *asic_internal_ss_get_ss_table(void *device)
+static ATOM_ASIC_INTERNAL_SS_INFO *asic_internal_ss_get_ss_table(void *device)
 {
 	ATOM_ASIC_INTERNAL_SS_INFO *table = NULL;
 	u8 frev, crev;