disp: msm: sde: check for null topology in dither setup path

Check for invalid topology value before extracting the number
of LMs in dither setup path. This occurs when there is a race
condition between runtime supend/resume and dynamic FPS switch.

Change-Id: Ibf4bd2fe5f094ddec11ebc5b85ea9a5cfa0e91a7
Signed-off-by: Amine Najahi <anajahi@codeaurora.org>