msm: mdss: Allow DMA pipe sharing only on WB mixer

DMA pipe can only be shared on the writeback mixer for
concurrent line and block mode operations. Only allow
sharing if the mixer attached to the pipe is already a
writeback mixer.

Change-Id: I9a0e5144fdf87f681493cd92d9cc809836ab1a5c
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>