drm/i915: Relocate intel_attached_dp()

We have uses for intel_attached_dp() outside of intel_dp.c. Move
it to a header.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204180549.1267-7-ville.syrjala@linux.intel.com


Reviewed-by: Mika Kahola <mika.kahola@intel.com>