drm/i915/guc: Tidy ELSP port assignment

Since we know that the port is empty, we do not need to extract the
count from the old request it and copy it over to the new request, or
attempt to unref the NULL old request pointer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171124130031.20761-1-chris@chris-wilson.co.uk