drm/tests: probe_helper: Fix uninitialized variable

The len variable is used while uninitialized. Initialize it.

Fixes: 1e4a91db ("drm/probe-helper: Provide a TV get_modes helper")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20221201090736.290935-1-maxime@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>