drm/i915: Avoid confusion between DP and TRANS_DP_CTL in DP .get_config()

Use a separate variable for the TRANS_DP_CTL value instead of reusing
'tmp' that otherwise contains the DP port register value.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>