drm/vmwgfx: Rename vmw_buffer_object to vmw_bo

The rest of the drivers which are using ttm have mostly standardized on
driver_prefix_bo as the name for subclasses of the TTM buffer object.
Make vmwgfx match the rest of the drivers and follow the same naming
semantics.

This is especially clear given that the name of the file in which the
object was defined is vmw_bo.c.

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/20230131033542.953249-4-zack@kde.org