disp:msm: fix return type from runtime api

pm_runtime_get_sync api can return non-zero return value
for success case. For such cases the IOCTL should return
success.

Change-Id: I5868cfc3c0cdeb63c3b2e68f1dcddadcd0aae9ce
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>