drm/amd/display: mark amdgpu_dm_connector_funcs_force static

A global function without a header prototype has made it into
linux-next during the merge window:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:6339:6: error: no previous prototype for 'amdgpu_dm_connector_funcs_force' [-Werror=missing-prototypes]

Mark the function static instead, as there are no other
callers outside this file.

Fixes: 0ba4a784

 ("drm/amd/display: implement force function in amdgpu_dm_connector_funcs")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202304251640.JClqTim9-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>