anv: allow NV12 <--> AHARDWAREBUFFER_FORMAT_Y8Cb8Cr8_420 inter-op

Merged Gurchetan Singh requested to merge gurchetansingh/mesa:flexformat-anv into master

AHARDWAREBUFFER_FORMAT_Y8Cb8Cr8_420 is an implementation defined flexible YUV format.  Most of the times, it's NV12 or YV12. On Intel, NV12 is preferred since it can be used by the display engine.  

This API adds a dependency between gralloc and buffer consumers, unfortunately. Right now, the code seems to work for i915 gralloc, but not cros_gralloc. Add a preprocessor flag to fix this.

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