drm/msm/sde: Fix a uninitalized variable compiler warning

The gcc compiler 6.1.1 complains that 'ret' is uninitalized in
_sde_encoder_rsc_client_update_vsync_wait.

Change-Id: Ic0dedbad3347552a73abe3fc6e2cfc2eda4ed391
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>