drm/i915/dp: make is_edp non-static and rename to intel_dp_is_edp

Expose across driver for future work. No functional changes.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818093020.19160-2-jani.nikula@intel.com