msm: mdss: delay overlay start until first update for external

Delay hardware initialization in the case of external display
until after continuous splash screen is completed. This delay
ensures iommu attach, clock update and bandwidth voting do not
happen until splash hand off is completed for primary display.

Change-Id: Ic974a748ee03dbc4cd690572841140e3eac48c39
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>