drm/i915: move intel_ddi_set_fia_lane_count to intel_tc.c

PORT_TX_DFLEXDPMLE1 is a FIA register so move it to intel_tc.c where we
access other FIA registers. In Tiger Lake we have multiple/modular FIAs
so it makes sense to start moving all access to their registers to a
common place.

While at it, make it clear that we will only ever call this function
for ports with TC phy. Previously we were relying on tc_mode being
TC_PORT_TBT_ALT for combo phy ports. However it's confusing since in
this same function we have checks for is_tc_port. Also, if we manage to
make each phy access only their own field, we may in future add them as
a union inside intel_digital_port.

v2: Fix coding style while moving the code

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708172815.6814-4-lucas.demarchi@intel.com