drm/panthor: Cleanup unused variable 'cookie'

Commit 962f88b9 ("drm/panthor: Drop the dev_enter/exit() sections in
_irq_suspend/resume()") removed the code that used the 'cookie' variable
but left the declaration in place. Remove it.

Fixes: 962f88b9 ("drm/panthor: Drop the dev_enter/exit() sections in _irq_suspend/resume()")
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Steven Price <steven.price@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402215423.360341-1-liviu.dudau@arm.com