Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/i915/display/intel_ddi.c: In function 'intel_ddi_get_config':
drivers/gpu/drm/i915/display/intel_ddi.c:3774:29: warning:
 variable 'intel_dig_port' set but not used [-Wunused-but-set-variable]
  struct intel_digital_port *intel_dig_port;

It is never used, so can be removed.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190705113138.65880-1-yuehaibing@huawei.com