[Why]
We're sending the drm vblank event a frame too early in the case where
the pageflip happens close to VUPDATE and ends up blocking the signal.

The implementation in DM was previously correct *before* we started
sending vblank events from VSTARTUP unconditionally to handle cases
where HUBP was off, OTG was ON and userspace was still requesting some
DRM planes enabled. As part of that patch series we dropped VUPDATE
since it was deemed close enough to VSTARTUP, but there's a key
difference betweeen VSTARTUP and VUPDATE - the VUPDATE signal can be
blocked if we're holding the pipe lock.

There was a fix recently to revert the unconditional behavior for the
DCN VSTARTUP vblank event since it was sending the pageflip event on
the wrong frame - once again, due to blocking VUPDATE and having the
address start scanning out two frames later.

The problem with this fix is it didn't update the logic that calls
drm_crtc_handle_vblank(), so the timestamps are totally bogus now.

[How]
Essentially reverts most of the original VSTARTUP series but retains
the behavior to send back events when active planes == 0.

Some refactoring/cleanup was done to not have duplicated code in both
the handlers.

Fixes: 16f17eda ("drm/amd/display: Send vblank and user events at vsartup for DCN")
Fixes: 3a2ce8d6 ("drm/amd/display: Disable VUpdate interrupt for DCN hardware")
Fixes: 2b5aed9a ("drm/amd/display: Fix pageflip event race condition for DCN.")
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-and-Tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 5.6.x