drm/i915: Actually flush interrupts on reset not just wedging

Commit 0f36a85c ("drm/i915: Flush pending interrupt following a GPU
reset") got confused and only applied the flush to the set-wedge path
(which itself is proving troublesome), but we also need the
serialisation on the regular reset path. Oops.

Move the interrupt into reset_irq() and make it common to the reset and
final set-wedge.

v2: reset_irq() after port cancellation, as we assert that
execlists->active is sane for cancellation (and is being reset by
reset_irq).

References: 0f36a85c ("drm/i915: Flush pending interrupt following a GPU reset")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Jeff McGee <jeff.mcgee@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180323101824.14645-1-chris@chris-wilson.co.uk