drm: Add Y2xx and Y4xx (xx:10/12/16) format definitions and fourcc
Unfortunately gcc didn't warn about the redefinitions, because the
double defines were the set to same value, and gcc apparently no longer
warns about that.
Fix this by using new XYVU for i915, without alpha, and making the
Y41x definitions match msdn, with alpha.
Fortunately we caught it early, and the conflict hasn't even landed in
drm-next yet.