drm/vmwgfx: Remove no-op prepare/cleanup_fb callbacks

Less hits to go through when I git grep over all drivers. These
callbacks are optional.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405154449.23038-2-daniel.vetter@ffwll.ch