drm/vmwgfx: Simplify fb pinning

Only the legacy display unit requires pinning of the fb memory in vram.
Both the screen objects and screen targets can present from any buffer.
That makes the pinning abstraction pointless. Simplify all of the code
and move it to the legacy display unit, the only place that needs it.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Maaz Mombasawala <mombasawalam@vmware.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230131033542.953249-5-zack@kde.org