drm/vmwgfx: Add implicit framebuffer checks to the screen target code

drm/vmwgfx: Add implicit framebuffer checks to the screen target code

Just like for screen objects, make sure we use only a single framebuffer
for implicit placement.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>