drm/i915: add CRC #defines for ilk/snb

Also add a new _PIPE_INC macro which takes an base plus increment.
Much less likely to botch the job by missing an s/A/B/ somewhere.

v2: They've moved the bitfield. Argh!

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>