drm/i915: Spinlocks in tasklets can use spin_(un)lock_irq

The tasklets callbacks are only called from tasklet context so
it is safe do to this.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170321105511.18269-1-tvrtko.ursulin@linux.intel.com