msm: mdss: fix the wrong register for the dma pipe on 8x10

During the LK to kernel transition, the driver checks the
frame buffer address configured to make sure it matches with
the kernel reserved address.   The frame buffer address should
be read from the dma-p pipe, instead of dma-s pipe.

Change-Id: Ic7c31d3166718ced2d2fb0c0591b2a2797e4e8d4
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>