msm: mdss: fix a warning for unused value

Value computed is not being used in the statement which is causing
a compilation error on some build flavors. Fix the issue by not
computing the value as it is not needed.

Change-Id: Ia628388deb3057da45989d6376503048b08e8167
Signed-off-by: Ashish Garg <ashigarg@codeaurora.org>