drm/i915: Expose latest 200 CRC value for pipe through debugfs

There are several points in the display pipeline where CRCs can be
computed on the bits flowing there. For instance, it's usually possible
to compute the CRCs of the primary plane, the sprite plane or the CRCs
of the bits after the panel fitter (collectively called pipe CRCs).

v2: Quite a bit of rework here and there (Damien)

Signed-off-by: Shuang He <shuang.he@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Fix intermediate compile file reported by Wu Fengguang's
kernel builder.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>