i915: make inject_virtual_interrupt() void

The single caller of inject_virtual_interrupt() ignores the return value
anyway. This allows us to simplify eventfd_signal() in follow-up
patches.

Link: https://lore.kernel.org/r/20231122-vfs-eventfd-signal-v2-1-bd549b14ce0c@kernel.org


Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>