drm/v3d: Document cache flushing ABI.

Right now, userspace doesn't do any L2T writes, but we should lay out
our expectations for how it works.

v2: Explicitly mention the VCD cache flushing requirements and that
    we'll flush the other caches before each of the CLs.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203222438.25417-1-eric@anholt.net


Reviewed-by: Dave Emett <david.emett@broadcom.com>