drm/i915: Tidy reporting busy status during i915_gem_retire_requests()

As we know by inspection whether any engine is still busy as we retire
all the requests, we can pass that information back via return value
rather than check again afterwards.

v2: A little more polish missed in patch splitting

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160827075401.16470-1-chris@chris-wilson.co.uk