Revert "drm/msm: rework vblank event handling in dpu_crtc"

This reverts commit 2f2eb723.

This reversion is happening as a result of disabling the
disp/dpu directory as it will not be used in 4.19 downstream
display driver and these changes are made in the common layer
which affects the downstream driver functionalities. Separate
patches are submitted to revert the upstream driver and
introduce the downstream version of display driver.

Change-Id: I740f5d8ed6afa5a1c2d9f59748bb796a9bb08e79
Signed-off-by: Samantha Tran <samtran@codeaurora.org>