drm/panthor: Avoid sleep locking in the internal BO size path

Commit 434e5ca5 ("drm/panthor: Expose size of driver internal BO's over
fdinfo") locks the VMS xarray, to avoid UAF errors when the same VM is
being concurrently destroyed by another thread. However, that puts the
current thread in atomic context, which means taking the VMS' heap locks
will trigger a warning as the thread is no longer allowed to sleep.

Because in this case replacing the heap mutex with a spinlock isn't
feasible, the fdinfo handler no longer traverses the list of heaps for
every single VM associated with an open DRM file. Instead, when a new heap
chunk is allocated, its size is accumulated into a pool-wide tally, which
also makes the atomic context code path somewhat faster.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Fixes: 434e5ca5 ("drm/panthor: Expose size of driver internal BO's over fdinfo")
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/20250303190923.1639985-2-adrian.larumbe@collabora.com