msm: mdss: use the dma pipe assigned from client

The dma pipe selection is done by client. Earlier,
the driver was trying to allocate a free dma pipe and check
if the allocated pipe matching with the selection.  Instead,
it is now assigning the pipe directly.

Change-Id: Ie3f239a02b0da1e598efe755329c1e5d9746166f
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>