drm/panthor: Lock XArray when getting entries for the VM

Similar to commit cac07570 ("drm/panthor: Fix race when converting
group handle to group object") we need to use the XArray's internal
locking when retrieving a vm pointer from there.

v2: Removed part of the patch that was trying to protect fetching
the heap pointer from XArray, as that operation is protected by
the @pool->lock.

Fixes: 647810ec ("drm/panthor: Add the MMU/VM logical block")
Reported-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241106185806.389089-1-liviu.dudau@arm.com