drm/i915: Clear engine irq posted following a reset

When the GPU is reset, we want to discard all pending notifications as
either we have manually completed them, or they are no longer
applicable. Make sure we do reset the engine->irq_posted prior to
re-enabling the engine (e.g. the interrupt tasklets) in
i915_gem_reset_finish_engine().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170721123238.16428-11-chris@chris-wilson.co.uk
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>