drm/i915/icl: implement icl_digital_port_connected()

drm/i915/icl: implement icl_digital_port_connected()

Do like the other functions and check for the status bits. The "Hot
Plug Detection" page from our documentation says we can't just use the
ISR bits on the CPU side (North Display, which has the TC and TBT
modes), so use the correct register: DFLEXDPSP, TC Live State field.

v2: Rebase.
v3:
  - Simplify true/false assignment (Rodrigo).
  - Reorganize is_gen if ladder (Rodrigo).
  - Don't use the ISR for TC/TBT CPU bits.
v4:
  - Improve commit message wording (Lucas).
v5:
  - COMMIT_LOG_LONG_LINE (Checkpatch).

Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com> (v3).
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180725195927.12059-1-paulo.r.zanoni@intel.com