drm/panthor: Fix access to uninitialized variable in tick_ctx_cleanup()

drm/panthor: Fix access to uninitialized variable in tick_ctx_cleanup()

commit 282864cc5d3f144af0cdea1868ee2dc2c5110f0d upstream.

The group variable can't be used to retrieve ptdev in our second loop,
because it points to the previously iterated list_head, not a valid
group. Get the ptdev object from the scheduler instead.

Cc: <stable@vger.kernel.org>
Fixes: d72f049087d4 ("drm/panthor: Allow driver compilation")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Closes: https://lore.kernel.org/r/202409302306.UDikqa03-lkp@intel.com/
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930163742.87036-1-boris.brezillon@collabora.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>