drm/i915: Introduce new intel_output_name()

That we can use for debugging purposes.

v2: Use designated initializers for the 'names' array (Paulo Zanoni,
    Jani Nikula).
    Add a check in case the array has a hole (which can now remain
    unnoticed with designated initializers) (Jani Nikula)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (for v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>