drm/i915/dp: rename intel_dp_is_edp to intel_dp_is_port_edp

Emphasize that this is based on the port, not intel_dp. This is also in
line with the underlying intel_bios_is_port_edp() function. 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/20170818093020.19160-1-jani.nikula@intel.com