drm: msm: dsi-staging: add support for LCD mode select GPIO

Certian DSI panels support both single/dual port modes based on
whether it is configured in split DSI or single DSI mode with
DSC enabled. The LCD mode select GPIO is needed to configure the
panel accordingly. Add support for this GPIO.

CRs-Fixed: 2008002
Change-Id: Ib96143d08ffb395f6fa6acd9e14d213b2bd32cae
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>