drm/i915/gt: Cleanup interface for MCR operations

Let's replace the assortment of intel_gt_* and intel_uncore_* functions
that operate on MCR registers with a cleaner set of interfaces:

  * intel_gt_mcr_read -- unicast read from specific instance
  * intel_gt_mcr_read_any[_fw] -- unicast read from any non-terminated
    instance
  * intel_gt_mcr_unicast_write -- unicast write to specific instance
  * intel_gt_mcr_multicast_write[_fw] -- multicast write to all instances

We'll also replace the historic "slice" and "subslice" terminology with
"group" and "instance" to match the documentation for more recent
platforms; these days MCR steering applies to more types of replication
than just slice/subslice.

v2:
 - Reference the new kerneldoc from i915.rst.  (Jani)
 - Tweak the wording of the documentation for a couple functions to
   clarify the difference between "_fw" and non-"_fw" forms.

v3:
 - s/read/write/ to fix copy-paste mistake in a couple comments.
   (Harish)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Harish Chegondi <harish.chegondi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220615001019.1821989-3-matthew.d.roper@intel.com