docs/panfrost: Document drm-shim usage
Document how to build and use Panfrost's drm-shim implementation. I hope by documenting this process, other Mesa developers are better able to test Panfrost. In particular, this allows developers without Mali hardware to run shader-db for any Mali target, which may be useful for debugging regressions from common NIR changes.
drm-shim is not a substitute for testing against real hardware.
Special thanks to Emma Anholt and Icecream95 for building this infrastructure.
Signed-off-by: Alyssa Rosenzweig alyssa@collabora.com