drm/i915: Use full serialisation around engine->irq_posted

Using engine->irq_posted for execlists, we are not always serialised by
the tasklet as we supposed. On the reset paths, the tasklet is disabled
and ignored. Instead, we manipulate the engine->irq_posted directly to
account for the reset, but if an interrupt fired before the reset and so
wrote to engine->irq_posted, that write may not be flushed from the
local CPU's cacheline until much later as the tasklet is already active
and so does not generate a mb(). To correctly serialise the interrupt
with reset, we need serialisation on the set_bit() itself.

And at last Mika can be happy.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
CC: Michel Thierry <michel.thierry@intel.com>
Cc: Jeff McGee <jeff.mcgee@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Jeff McGee <jeff.mcgee@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322073533.5313-3-chris@chris-wilson.co.uk