drm/i915/display: prefer dig_port to reference intel_digital_port

We have a mix of dport, intel_dport, intel_dig_port and dig_port to
reference a intel_digital_port struct. Numbers are around

5	intel_dport
36	dport
479	intel_dig_port
352	dig_port

Since we already removed the intel_ prefix from most of our other
structs, do the same here and prefer dig_port.

v2: rename everything in i915, not just a few display sources and
reword commit message (from Matt Roper)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701045054.23357-1-lucas.demarchi@intel.com