Revert "drm/exynos: exynos7_drm_decon: add vblank check in IRQ handling"

This reverts commit b4e72c0b which is
commit b846350a upstream.

It breaks the Android kernel build and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I5036544ff96d39426f2c09cb10a68422d61cb98d
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>