drm/vmwgfx: Fix up the implicit display unit handling

Make the connector is_implicit property immutable.
As far as we know, no user-space application is writing to it.

Also move the verification that all implicit display units scan out
from the same framebuffer to atomic_check().

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