drm/i915/execlists: Reduce completed event mask to COMPLETE | PREEMPTED

Since we get a COMPLETE event when the context switch occurs on
RING_HEAD == RING_TAIL and a PREEMPTED event when a switch occurs
before that point, COMPLETE | PREEMPTED should cover all possible context
switch completion events. We can move the ELEMENT_SWITCH info message
from the COMPLETED_MASK into an assertion for when we are performing a
switch to port[1].

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171120123458.23242-2-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>