drm/i915/gt: Initialize unused MOCS entries with device specific values

Historically we've initialized all undefined/reserved entries in
a platform's MOCS table to the contents of table entry #1 (i.e.,
I915_MOCS_PTE).
Going forward, we can't assume that table entry #1 will always
contain suitable values to use for undefined/reserved table
indices. We'll allow a platform-specific table index to be
selected at table initialization time in these cases.

This new mechanism to select L3 WB entry will be applicable for
all the Gen12+ platforms except TGL and RKL.

Since TGL and RLK are already in production so their mocs settings
are intact to avoid ABI break.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ayaz A Siddiqui <ayaz.siddiqui@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210903092153.535736-5-ayaz.siddiqui@intel.com