drm/i915: Return zero as the scanline counter for disabled pipes

We print the scanline counters as unsigned integers so the -1
here just makes the debugs/traces look a bit messy. Zero seems
equally valid for this usecase.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210304170421.10901-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>