drm/amd/display: Prevent OTG shutdown during PSR SU

drm/amd/display: Prevent OTG shutdown during PSR SU

[Why]

Enabling Z10 optimizations allows DMUB to disable the OTG during PSR
link-off. This theoretically saves power by putting more of the display
hardware to sleep. However, we observe that with PSR SU, it causes
visual artifacts, higher power usage, and potential system hang.

This is partly due to an odd behavior with the VStartup interrupt used
to signal DRM vblank events. If the OTG is toggled on/off during a PSR
link on/off cycle, the vstartup interrupt fires twice in quick
succession. This generates incorrectly timed vblank events.
Additionally, it can cause cursor updates to generate visual artifacts.

Note that this is not observed with PSR1 since PSR is fully disabled
when there are vblank event requestors. Cursor updates are also
artifact-free, likely because there are no selectively-updated (SU)
frames that can generate artifacts.

[How]

A potential solution is to disable z10 idle optimizations only when fast
updates (flips & cursor updates) are committed. A mechanism to do so
would require some thoughtful design. Let's just disable idle
optimizations for PSR2 for now.

Fixes: 7cc191ee7621 ("drm/amd/display: Implement MPO PSR SU")
Reported-by: August Wikerfors <git@augustwikerfors.se>
Link: https://lore.kernel.org/r/c1f8886a-5624-8f49-31b1-e42b6d20dcf5@augustwikerfors.se/
Tested-by: August Wikerfors <git@augustwikerfors.se>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>