msm: mdss: handle continuous splash screen for dual DSI cases

In the current implementation, the continuous splash screen flag
is set based on whether there is handoff pending in MDP or not.
With the dual DSI use-case now in place, there is possibility that
the primary panel has continuous splash enabled whereas the other
panel doesn't have the feature enabled. Add change to take care of
this by checking if a particular interface is enabled in MDP_INTF_SEL
register to set the cont. splash screen flag. Remove the DT entries
for continuous splash screen since it is no longer needed after this
change.

Change-Id: I4d617386c8f5d166de76b79a10680d024320a889
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>