msm: mdss: add MDSS_EVENT_LINK READY

At split display case, DSI panel initialization dcs
commands need to be sent to panel when both dsi links
are in LP-11 state. Add MDSS_EVENT_LINK_READY to have
both dsi controller's phy/pll initialized which brings
link to LP-11 state first so that panel initialization
dcs commands be received by panel successfully to bring
panel to display_on state.

Change-Id: I4b33fafd2673be1f068819bee25fc1a335eb0fe8
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>