drm/i915: Remove stale comment about intel_dp_detect()

The function doesn't use any of the registers mentioned, nor does it
return true or false. Hard to do worse. Remove it, the function is
absolutely descriptive enough to not need any comment.

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>