drm/i915: Make execlist port count variable

As we emulate execlists on top of the GuC workqueue, it is not
restricted to just 2 ports and we can increase that number arbitrarily
to trade-off queue depth (i.e. scheduling latency) against pipeline
bubbles.

v2: rebase. better commit msg (Chris)
v3: rebase

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170922124307.10914-5-mika.kuoppala@intel.com