drm/i915: Sanitize the terminology used for TypeC port modes

The TypeC port mode can switch dynamically, to reflect that better call
the port's mode as 'mode' rather than 'type'.

While at it:
- s/TC_PORT_TBT/TC_PORT_TBT_ALT/ and s/TC_PORT_TYPEC/TC_PORT_DP_ALT/.
  'TYPEC' is ambiguous, TBT_ALT and DP_ALT better match the reality.

- Remove the 'unknown' TypeC port mode. The mode is always known, it's
  the TBT-alt/safe mode after HW reset and after disconnecting the PHY.
  Simplify the tc_port/tc_type checks accordingly.

- Don't WARN if the port mode changes, that can happen normally.

No functional changes.

Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628143635.22066-5-imre.deak@intel.com