Ever since commit 204474a6 ("drm/i915: Pass down rc in
intel_encoder->compute_config()") we're supposed to return an
errno from .compute_config(). I failed to notice that when
pushing the TV encoder fixes which were written before said
commmit. Fix up the return value for the error case.

Cc: Imre Deak <imre.deak@intel.com>
Fixes: 690157f0 ("drm/i915/tv: Fix >1024 modes on gen3")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125181931.19482-1-ville.syrjala@linux.intel.comReviewed-by: Imre Deak <imre.deak@intel.com>