drm/panthor: Don't check the array stride on empty uobj arrays

The user is likely to leave all the drm_panthor_obj_array fields
to zero when the array is empty, which will cause an EINVAL failure.

v2:
- Added R-bs

Fixes: 4bdca115 ("drm/panthor: Add the driver frontend block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240703071640.231278-2-boris.brezillon@collabora.com