drm: msm: update dsi state context when splash is on

DSI phy state's update should be called by one unique
API, to ensure no operation is missed. Otherwise, the
incomplete operation will bring DSI regulator off error
and trigger device watchdog reset.

CRs-Fixed: 2282416
Change-Id: If8d08f224f713547a4df607f817e21c49bbc4ed6
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>