drm/msm: Construct only one encoder for DSI

We currently create 2 encoders for DSI interfaces, one for command
mode and other for video mode operation. This isn't needed as we
can't really use both the encoders at the same time. It also makes
connecting bridges harder.

Switch to creating a single encoder. For now, we assume that the
encoder is configured only in video mode. Later, the same encoder
would be usable in both modes.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>