platform: msm: Remove incorrect external display stream id assignment

The stream id for an external display is
currently being set to the total number of
external displays attached to the system. This
logic is incorrect and leads to issues when
displays are disconnected and reconnected while
there are other display(s) still connected.

Removing this assignment will cause the stream id
to take on the value set during the registration
of the display, which is the correct stream id.

Change-Id: Ia6e2a9c161b795bcf7bd984f0f5c64aa67c81c57
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>