drm/i915: No need to save/restore irq status in intel_engine_wakeup

It is called from either the process or timer context so it is
correct to always disable interrupts.

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/20170306150321.29024-1-tvrtko.ursulin@linux.intel.com