drm/vmwgfx: Switch to a managed drm device

To cleanup some of the error handling and prepare for some
other work lets switch to a managed drm device. It will
let us get a better handle on some of the error paths.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/414039/?series=85516&rev=2