drm/i915: Serialize per-engine resets against new requests

We rely on disabling the execlists (by stopping the tasklet) to prevent
new requests from submitting to the engine ELSP before we are ready.
However, we re-enable the engine before we call init_hw which gives
userspace the opportunity to subit a new request which is then
overwritten by init_hw -- but not before the HW may have started
executing. The subsequent out-of-order CSB is detected by our sanity
checks in intel_lrc_irq_handler().

Fixes: a1ef70e1

 ("drm/i915: Add support for per engine reset recovery")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170721123238.16428-3-chris@chris-wilson.co.uk
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>