drm/vc4: Add get/set tiling ioctls.

This allows mesa to set the tiling format for a BO and have that
tiling format be respected by mesa on the other side of an
import/export (and by vc4 scanout in the kernel), without defining a
protocol to pass the tiling through userspace.

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