drm/amd/display: check dc_link before dereferencing

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:6683 amdgpu_dm_connector_funcs_force()
warn: variable dereferenced before check 'dc_link' (see line 6663)

Fixes: 96717617 ("drm/amd/display: fix null-pointer dereference on edid reading")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>