drm/panthor: Don't add write fences to the shared BOs

drm/panthor: Don't add write fences to the shared BOs

commit f9e7ac6e2e9986c2ee63224992cb5c8276e46b2a upstream.

The only user (the mesa gallium driver) is already assuming explicit
synchronization and doing the export/import dance on shared BOs. The
only reason we were registering ourselves as writers on external BOs
is because Xe, which was the reference back when we developed Panthor,
was doing so. Turns out Xe was wrong, and we really want bookkeep on
all registered fences, so userspace can explicitly upgrade those to
read/write when needed.

Fixes: 4bdca1150792 ("drm/panthor: Add the driver frontend block")
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240905070155.3254011-1-boris.brezillon@collabora.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>