drm/msm: refactor Y420CMDB block parsing logic

Y420CMDB block parsing is too complex and incorrect
in handling cases where more than one byte of bitmap
is present.

Fix this logic to make it more simple and capable to
handle all sucn cases.

Change-Id: I7aef80f588ec44def000c9f04e1da4c10020699d
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>