msm: mdss: use one dma pipe during block mode transaction

Use one dma pipe during block mode transaction instead of two pipes
for the buffer resolution greater than max mixer width

Change-Id: I2a9f1bba95bfb66d1c0168aa9b99b8b9de57cd6a
Signed-off-by: Ramkumar Radhakrishnan <ramkumar@codeaurora.org>