drm/i915: intel_ring_initialized() must be simple and inline

Based on Chris Wilson's patch from 6 months ago, rebased and adapted.

The current implementation of intel_ring_initialized() is too heavyweight;
it's a non-inlined function that chases several levels of pointers. This
wouldn't matter too much if it were rarely called, but it's used inside
the iterator test of for_each_ring() and is therefore called quite
frequently. So let's make it simple and inline ...

The idea here is to use ring->dev as an indicator showing which engines
have been initialised and are therefore to be included in iterations that
use for_each_ring(). This allows us to avoid multiple memory references
and a (non-inlined) function call on each iteration of each such loop.

	Fixes regression from
	commit 48d82387
	Author: Oscar Mateo <oscar.mateo@intel.com>
	Date:   Thu Jul 24 17:04:23 2014 +0100

	    drm/i915/bdw: Generic logical ring init and cleanup

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449586956-32360-2-git-send-email-david.s.gordon@intel.com