drm/i915: extract dri1 breadcrumb update from irq handler
... and hide it in i915_dma.c.
This way all the legacy stuff dealing with READ_BREADCRUMB and
LP_RING and friends is in i915_dma.c.
v2: Rebase on top of Chris Wilson's rework irq handling code.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>