drm/exynos: allow multiple layer updates per vsync for mixer

Allowing only one layer update per vsync can cause issues
while there are update available for both layers. There is
a good amount of possibility to loose updates if we allow
single update per vsync.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>