drm/amdgpu: declare static function to fix compiler warning

>> drivers/gpu/drm/amd/amdgpu/amdgpu_psp.c:503:6: warning: no previous prototype for function 'release_psp_cmd_buf' [-Wmissing-prototypes]
   void release_psp_cmd_buf(struct psp_context *psp)
        ^
   drivers/gpu/drm/amd/amdgpu/amdgpu_psp.c:503:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
   void release_psp_cmd_buf(struct psp_context *psp)
   ^
   static
   1 warning generated.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Kevin Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>