drm/i915/dp: Use a local variable for intel_encoder *

We have two cases of intel_dp to intel_encoder conversions, use a
local variable to store the conversion.

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/20180927205735.16651-3-dhinakaran.pandiyan@intel.com