drm/xe/xe2: Add performance tuning for L3 cache flushing

A recommended performance tuning for LNL related to L3 cache flushing
was recently introduced in Bspec. Implement it.

Unlike the other existing tuning settings, we limit this one for LNL
only, since there is no info about whether this would be applicable to
other platforms yet. In the future we can come back and use IP version
ranges if applicable.

v2:
  - Fix reference to Bspec. (Sai Teja, Tejas)
  - Use correct register name for "Tuning: L3 RW flush all Cache". (Sai
    Teja)
  - Use SCRATCH3_LBCF (with the underscore) for better readability.
v3:
  - Limit setting to LNL only. (Matt)

Bspec: 72161
Cc: Sai Teja Pottumuttu <sai.teja.pottumuttu@intel.com>
Cc: Tejas Upadhyay <tejas.upadhyay@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240920211459.255181-5-gustavo.sousa@intel.com