msm: mdss: Use MDSS reg for version

MDSS and MDP regs have same version value for all the targets
other than mpq8092. In mpq, MDSS version is different, so using
it instead of MDP version to get the unique value, and using this
new version to check correct write-back format.

Change-Id: Ide7335815d61681c7a6909b9da95d0b6f4851d54
Signed-off-by: Zohaib Alam <zalam@codeaurora.org>