drm/i915/icl: use tc_port in MG_PLL macros

Fix the TODO leftover in the code by changing the argument in MG_PLL
macros. The MG_PLL ids used to access the register values can be
converted from tc_port rather than port.

All these registers can use the TC port to calculate the right offsets
because they are only available for TC ports. The range (PORT_C onwards)
may not be stable and change from platform to platform. So by using the
TC id directly we avoid having to check for the platform in the "leaf
functions" and thus passing dev_priv around.

The helper functions were also renamed to use "tc" as prefix to make
them more generic.

v2: Improve commit message and fix checkpatch warning (from Paulo)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125222444.19926-2-lucas.demarchi@intel.com