drm/i915/dp: Fix duplication of DEVICE_SERVICE_IRQ handling

There are two copies of the same code called from long and short
pulse handlers.

v2: Rebase due to s/int status/enum drm_connector_status in
intel_dp_detect()
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180927205735.16651-6-dhinakaran.pandiyan@intel.com