drm/xe: Never report L3 bank mask for media GT going forward

We currently report an L3 bank mask as part of the GT topology on both
GTs (primary and media) because a copy of the L3 bank fuse register
exists on both GTs (e.g., $gsi_offset + 0x9130 on Xe3).  After recent
discussions it's come to light that the only known userspace software
that uses this part of the uapi (the compute UMD and Mesa) only uses the
value reported for the primary GT; the value reported for the media GT
is ignored by both projects, and the media UMDs don't have any use for
L3 information today.  Since we always strive to have our uapi match the
specific needs of userspace and not include additional unused baggage,
let's officially drop L3 bank reporting on the media GT going forward
and only keep it around for the primary GT where it actually gets used.
This change will only apply to future platforms (Xe3 and later); even
though it would probably be safe to remove it from Xe1/Xe2 as well, we
don't want to take any chances with changing existing ABI.

Note that we'd already disabled reading/reporting of the L3 bank for the
media GT on PTL in commit 9ab440a9d042 ("drm/xe/ptl: L3bank mask is not
available on the media GT") because it was discovered that the copy of
the fuse registers on the media GT were just reporting a bogus ~0 value
rather than an accurate mask.  So this is just extending that PTL
behavior forward to WCL and other future platforms.  Note that we're
also free to reinstate this part of the uapi in the future if/when some
new userspace consumer emerges that _does_ have a use for media-specific
L3 bank masks.

Cc: Fei Yang <fei.yang@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/20250905215614.796247-3-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>

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