drm/i915/execlists: Kick start request processing after a reset

drm/i915/execlists: Kick start request processing after a reset

During a reset, we may skip over completed requests and lost
context-switch interrupts. Following the reset, we may then may end up
with no active requests in the ELSP (and so do not resubmit to restart
the engine), but have a queue of requests ready for execution. This is
unlikely, it requires the last request to complete after the hang is
detected, but not impossible. The outcome of this is that the engine
stalls, possibly leading to full ring and indefinite wait under
struct_mutex, eventually leading to a full driver hang.

Alternatively, we can solve this by unsubmitting the incomplete requests
and just kickstarting the tasklet. Michał has patches for that, which I
initially disliked due to the extra complexity, but the complexity of
this "simple" restart is growing...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170916204414.32762-1-chris@chris-wilson.co.uk
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>