drm/i915/perf: drop list of streams

At some point in time there was the idea that we could have multiple
stream from the same piece of HW but that never materialized and given
the hard time we already have making everything work with the
submission side, there is no real point having this list of 1 element
around.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@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/20191008140111.5437-1-chris@chris-wilson.co.uk