drm/i915: Add modular FIA

Some platforms may have Modular FIA. If Modular FIA is used in the SOC,
then Display Driver will access the additional instances of
FIA based on pre-assigned offset in GTTMADDR space.

Each Modular FIA instance has its own IOSF Sideband Port ID
and it houses only 2 Type-C Port. In SOC that has more than
two Type-C Ports, there are multiple instances of Modular FIA.
Gunit will need to use different destination ID when it access
different pair of Type-C Port.

The DFLEXDPSP register has Modular FIA bit starting on Tiger Lake.  If
Modular FIA is used in the SOC, this register bit exists in all the
instances of Modular FIA. IOM FW is required to program only the MF bit
in first FIA instance that houses the Type-C Port 0 and Port 1, for
Display Driver to read from.

v2 (Lucas):
  - Move all accesses to FIA to be contained in intel_tc.c, along with
    display_fia that is now called tc_phy_fia
  - Save the fia instance number on intel_digital_port, so we don't have
    to query if modular FIA is used on every access
v3 (Lucas): Make function static
v4 (Lucas): Move enum phy_fia to the header and use it in
   intel_digital_port (suggested by Ville)
v5 (Lucas): Add comment about the mapping between FIA and TC port
   (suggested by Stuart)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712055706.12143-2-lucas.demarchi@intel.com