drm/xe/dma-buf: Allow pinning of p2p dma-buf

RDMA NICs typically requires the VRAM dma-bufs to be pinned in
VRAM for pcie-p2p communication, since they don't fully support
the move_notify() scheme. We would like to support that.

However allowing unaccounted pinning of VRAM creates a DOS vector
so up until now we haven't allowed it.

However with cgroups support in TTM, the amount of VRAM allocated
to a cgroup can be limited, and since also the pinned memory is
accounted as allocated VRAM we should be safe.

An analogy with system memory can be made if we observe the
similarity with kernel system memory that is allocated as the
result of user-space action and that is accounted using __GFP_ACCOUNT.

Ideally, to be more flexible, we would add a "pinned_memory",
or possibly "kernel_memory" limit to the dmem cgroups controller,
that would additionally limit the memory that is pinned in this way.
If we let that limit default to the dmem::max limit we can
introduce that without needing to care about regressions.

Considering that we already pin VRAM in this way for at least
page-table memory and LRC memory, and the above path to greater
flexibility, allow this also for dma-bufs.

v2:
- Update comments about pinning in the dma-buf kunit test
  (Niranjana Vishwanathapura)

Cc: Dave Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://lore.kernel.org/r/20250918092207.54472-4-thomas.hellstrom@linux.intel.com

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