mdss: share fb device's primary vs secondary selection info

FrameBuffer device clients assume that first FB device (fb0)
is primary and rest are secondary/external. This binds the
fb devices to fix probe order. For Example: Assumption breaks
when DSI probes first but selected as secondary interface and
HDMI probes later but selected as primary interface. Ideally,
MDSS driver should share each device's primary vs secondary
interface selection with client. This change fixes it.

Change-Id: I17ae8fef9604edf60ddbe22f8413d6764a5a1be4
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>