drm/i915: Factor out common parts from TypeC port handling functions

Factor out helpers reading/parsing the TypeC specific registers, making
current users of them clearer and letting us use them later.

While at it also:
- Simplify icl_tc_phy_connect() with an early return in legacy mode.
- Simplify the live status check using one bitmask for all HPD bits.
- Remove a micro-optimisation of the repeated safe-mode clearing.
- Make sure we fix the legacy port flag in all cases.

Except for the last two, no functional changes.

v2:
- Don't do reg reads at variable declarations. (Jani)
- Prevent constant truncated compiler warning when assigning the
  valid_hpd_mask. (Nick)
- s/intel_tc_port_get_lane_info/intel_tc_port_get_lane_mask/ (Ville)
v3:
- Make valid_hpd_mask init clear. (Ville)

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628143635.22066-10-imre.deak@intel.com