msm: mdss: fix smp calculation for yuv frames

Shared memory pool calculations should be based on the width of the
surface when calculating the stride of second plane. This may lead to
unexpected SMP allocations leading to underflow or waste of SMP blocks.

Change-Id: I4eb998cff4f05273b7aa0111c917fa151b71b140
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>