drm/i915/display: stop returning errors from debugfs registration

Failures to register debugfs should be ignored anyway, so stop
propagating errors altogether for clarity and simplicity. No functional
changes.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/346562ccef2282ccdbdea54409fab1d2b48f313c.1630327990.git.jani.nikula@intel.com