drm/i915/execlists: Split the atomic test_and_clear_bit for irq handler

drm/i915/execlists: Split the atomic test_and_clear_bit for irq handler

Rather than impose the cost of a locked test before queuing a new
request, reduce it to a simple test_bit() with a following clear_bit()
prior to doing the CSB check. This ensure that if an interrupt does
occur whilst reading from the CSB, we still detect it (the interrupt
would trigger a rescheduling of the tasklet anyway).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170321113320.2603-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>