drm/i915/tgl: Initialize multicast register steering for workarounds

Even though the bspec is missing gen12 register details for the MCR
selector register (0xFDC), this is confirmed by hardware folks to be a
mistake; the register does exist and we do indeed need to steer
multicast register reads to an appropriate instance the same as we did
on gen11.

Note that despite the lack of documentation we were still using the MCR
selector to read INSTDONE and such in read_subslice_reg() too.

Cc: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200414211118.2787489-4-matthew.d.roper@intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>

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