drm/vmwgfx: Remove stealth mode

Before drm got helpers for removing conflicting pci framebuffer devices
we implemented something known as "stealth" mode which allowed vmwgfx
to run even if it couldn't reserve pci resources. We can just switch
to regular drm helpers instead of keeping the stealth mode alive as
it makes our code a lot cleaner.

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/414043/?series=85516&rev=2