drm/i915/display: remove alias to dig_port

We don't need intel_dig_port and dig_port to refer to the same thing.
Prefer the latter.

v2: fix coding style

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200626234834.26864-2-lucas.demarchi@intel.com