msm: mdss: Allow writeback panel to start overlay during handoff

There is no longer a need to delay the hardware initialization for WB.
Previously, iommu attach was done automatically in overlay_start,
which caused page faults. However, now the iommu is attached at the same
time as continuous splash handoff, which prevents any unwanted page
faults.

Change-Id: I79f021da1ada7e342b9db84167032df5ac840e4e
Signed-off-by: Benet Clark <benetc@codeaurora.org>