msm: mdss: Add one device attribute to expose pack pattern

Provide userspace the knowledge of packpattern using sysfs node
so that it could arrange R/G/B channel properly before using CC
block on DMA_P pipe.

Change-Id: Ib4f4768adf7a90c5355fb0494e32df290cc6b058
Signed-off-by: Lei Zhou <leizhou@codeaurora.org>