drm/vc4: Add suport for 3D rendering using the V3D engine.

This is a squash of the out-of-tree development series.  Since that
series contained code from the first "get a demo triangle rendered
using a hacked up driver using binary shader code" to "plug the last
known security hole", it's hard to reconstruct a different series of
incremental development that's mergeable without security holes
throughout it.

Signed-off-by: Eric Anholt <eric@anholt.net>