drm/i915: fix the dequeue logic for single_port_submission context

For a single_port_submission context, GVT expects that it can only be
submitted to port 0, and there shouldn't be any other context in port 1
at the same time. This is required by GVT-g context to have an opportunity
to save/restore some non-hw context render registers.

This patch is to workaround GVT-g.

v2: optimized code by following Chris's advice, and added more comments to
explain the patch.
v3: followed the coding style.

Signed-off-by: Min He <min.he@intel.com>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1479305104-17049-1-git-send-email-min.he@intel.com