drm/amd/display: Fixed kernel test robot warning

Kernel test robot throws below warning ->

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5349:5:
warning: no previous prototype for 'amdgpu_dm_crtc_atomic_set_property'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5349:5:
warning: no previous prototype for function
'amdgpu_dm_crtc_atomic_set_property' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5373:5:
warning: no previous prototype for 'amdgpu_dm_crtc_atomic_get_property'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:5373:5:
warning: no previous prototype for function
'amdgpu_dm_crtc_atomic_get_property' [-Wmissing-prototypes]

As these functions are only used inside amdgpu_dm.c, these can be
made static.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>