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

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

BugLink: https://bugs.launchpad.net/bugs/1763271
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
(cherry picked from commit 1853a9daa19e150c32d0082e4f006391a7c6a997)
Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>