drm/msm/sde: provide max prefetch default if not defined

Max prefetch is specified in device tree to indicate the
maximum prefetch lines for current architecture. Provide
default value to max prefetch if not specified in device
tree.

CRs-Fixed: 2048612
Change-Id: If28a993a60f5b24fa8077b981e8ca04d3f5de26b
Signed-off-by: Alan Kwong <akwong@codeaurora.org>