drm/i915/gem: use spinlock_t instead of struct spinlock

spinlock_t is one case where the typedef is to be preferred over struct
spinlock.

Fixes: 42fb60de

 ("drm/i915/gem: Don't leak non-persistent requests on changing engines")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200217184219.15325-1-jani.nikula@intel.com