kgsl/drm: add fake vsync for WB operation.

We need to support fake vsync for UI updating in the WB operation with
LCD off statement.

Change-Id: I564b979e51c8ee6dd6ad377e4d698a271f90780d
Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Mathew Karimpanal <kjmathew@codeaurora.org>
(cherry picked from commit 500bf1a4f1ae0084315b104a9cf305ab42261863)