drm/msm/dsi-staging: decouple label and display type for DSI

DSI driver requires label and display type to be the same,
which introduces the limitation that only DSI display can be the
primary display. This change will decouple the label and display
type to allow other display to set to primary.

Change-Id: Ibff404ddc9927c239420861af6bce025ced340cc
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>