drm/i915/icl: introduce tc_port

Add and enum for TC ports and auxiliary functions to handle them.
Icelake brings a lot of registers and other things that only apply to
the TC ports and are indexed starting from 0, so having an enum for
tc_ports that starts at 0 really helps the indexing.

This patch is based on previous patches written by Dhinakaran Pandiyan
and Mahesh Kumar.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180522002558.29262-4-paulo.r.zanoni@intel.com