drm/i915/hdcp: fix intel_hdcp_get_repeater_ctl() error return value

intel_hdcp_get_repeater_ctl() is supposed to return unsigned register
contents. Returning negative error values is unexpected, and none of the
callers check for that.

Sort of fix the error cases by returning 0. I don't think we should hit
these cases anyway, and using 0 for the registers is safer than
0xffffffea (-EINVAL).

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231219104746.1065431-2-jani.nikula@intel.com