drm/i915/perf: Fix OA context id overlap with idle context id

Engine context pinned in perf OA was set to same context id as
the idle context. Set the context id to an unused value.

Clear the sw context id field in lrc descriptor before ORing with
ce->tag (Chris)

Closes: https://gitlab.freedesktop.org/drm/intel/issues/756


Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200124013701.40609-1-umesh.nerlige.ramappa@intel.com