drm/amd/display: Fix no previous prototype warning

drm/amd/display: Fix no previous prototype warning

[Why]
Received compiling warning:

All warnings (new ones prefixed by >>):

>> drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5574:5:
warning: no previous prototype for 'amdgpu_dm_crtc_late_register'
[-Wmissing-prototypes]
    5574 | int amdgpu_dm_crtc_late_register(struct drm_crtc *crtc)
         |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
   drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c: In
function 'dm_update_mst_vcpi_slots_for_dsc':
   drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:6405:46:
warning: variable 'old_con_state' set but not used
[-Wunused-but-set-variable]
    6405 |  struct drm_connector_state *new_con_state, *old_con_state;
         |                                              ^~~~~~~~~~~~~
   drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c: In
function 'amdgpu_dm_commit_cursors':
   drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:8006:44:
warning: variable 'new_plane_state' set but not used
[-Wunused-but-set-variable]
    8006 |  struct drm_plane_state *old_plane_state, *new_plane_state;
         |                                            ^~~~~~~~~~~~~~~

vim +/amdgpu_dm_crtc_late_register +5574
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c

  5572
  5573  #ifdef CONFIG_DRM_AMD_SECURE_DISPLAY
> 5574  int amdgpu_dm_crtc_late_register(struct drm_crtc *crtc)
  5575  {
  5576          crtc_debugfs_init(crtc);
  5577
  5578          return 0;
  5579  }
  5580  #endif
  5581

[How]
Fix it with declaration as "static"

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>