drm/i915: Add intel_ring_cachline_align()

drm/i915: Add intel_ring_cachline_align()

commit 753b1ad4a281b0663329409d410243e91825c323 upstream.

intel_ring_cachline_align() emits MI_NOOPs until the ring tail is
aligned to a cacheline boundary.

Cc: Bjoern C <lkml@call-home.ch>
Cc: Alexandru DAMIAN <alexandru.damian@intel.com>
Cc: Enrico Tagliavini <enrico.tagliavini@gmail.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>