drm/i915/execlists: Microoptimise execlists_cancel_port_request()

Just rearrange the code slightly to trim the number of iterations
required.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170925124929.16974-1-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>