backend-drm: Abide by the weston-direct-display specification

Turns out it seems we have a missed some spots when using the
weston-direct-display protocol.

Specifically we still seem to be attempting a dma buffer import which
ultimately can reach the GPU. This patch rectifies that in such a way
that we can actually provide a full scanout path for client's (dma)
buffers as to avoid hitting the GPU entirely.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>