[Why]
We dropped the delay after changed the cursor functions locking the
entire pipe to locking just the CURSOR registers to fix page flip
stuttering - this introduced cursor stuttering instead, and an underflow
issue.

The cursor update can be delayed indefinitely if the cursor update
repeatedly happens right around VUPDATE.

The underflow issue can happen if we do a viewport update on a pipe
on the same frame where a cursor update happens around VUPDATE - the
old cursor registers are retained which can be in an invalid position.

This can cause a pipe hang and indefinite underflow.

[How]
The complex, ideal solution to the problem would be a software
triple buffering mechanism from the DM layer to program only one cursor
update per frame just before VUPDATE.

The simple workaround until we have that infrastructure in place is
this change - bring back the delay until VUPDATE before locking, but
with some corrections to the calculations.

This didn't work for all timings before because the calculation for
VUPDATE was wrong - it was using the offset from VSTARTUP instead and
didn't correctly handle the case where VUPDATE could be in the back
porch.

Add a new hardware sequencer function to use the existing helper to
calculate the real VUPDATE start and VUPDATE end - VUPDATE can last
multiple lines after all.

Change the udelay to incorporate the width of VUPDATE as well.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>