drm/exynos: clean up wait_for_vblank

This patch cleans up wait_for_vblank relevant codes.
wait_for_vblank callback isn't used anymore in Exynos drm driver
so it removes relevant codes. However, display controllers -
FIMD and DECON - still use this function driver internally
to ensure shadow registers to be updated, which resolves
page fault issue so keep it.

Signed-off-by: Inki Dae <inki.dae@samsung.com>