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: NThomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: NSinclair Yeh <syeh@vmware.com>
Reviewed-by: NDeepak Rawat <drawat@vmware.com>