mdss:display: add regulator control to dsi-1

Regulators are shared by both dsi controllers. For both
dual and split display application regulator should be
controlled individually to avoid turning them off while
dsi-1 is still running. Not doing so causes DLB0_ERR_CONTENTION_LP1
error

CRs-fixed: 579900
Change-Id: Ia29bd3e49a405ccb1ad55f08f3897cee2ff6f4c6
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>