drm/i915/sdvo: Make .get_modes() return the number of modes

.get_modes() is supposed to return the number of modes added to the
probed_modes list (not that anyone actually checks for anything
except zero vs. not zero). Let's do that. Also switch over to using
intel_connector_update_modes() instead of hand rolling it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200108181242.13650-8-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>