gpu: drm: i915: intel_display.c: Remove unused function

Remove the function intel_output_name() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>