msm: mdss: parse and populate PPB offsets separately

This change parses and populates PPB control and config register
offsets separately. Its not necessary every ping pong blocks
to have both control and config registers.

Change-Id: I4be0dcaa9fabbd81e4875255d808707bf1e97e8e
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>