msm: mdss: fix dma fifo read watermark to 15/16 full

Previously the dma fifo read watermark was set to default value
which is 1/2 full. The full FIFO size is 512B, and this means the
read would be blocked for the first 256B and as soon as it is
consumed, it might expect the next 256B immediately. This may
result in underflow if sufficient data is not produced. To avoid
such cases, set read watermark to 15/16, so it is blocked until
full FIFO size data is generated.

Change-Id: I39bd10ccb7db3c4fe37ea609babdd6305fc19a7d
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>