drm/meson: Use optional canvas provider

This is the first step into converting the meson/drm driver to use
the canvas module.

If a canvas provider node is detected in DT, use it. Otherwise,
fall back to what is currently being done.

Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: added back priv in meson_drv_unbind()]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181105104508.23090-3-mjourdan@baylibre.com