drm/i915/color: Stop using non-posted DSB writes for legacy LUT

drm/i915/color: Stop using non-posted DSB writes for legacy LUT

DSB LUT register writes vs. palette anti-collision logic
appear to interact in interesting ways:
- posted DSB writes simply vanish into thin air while
  anti-collision is active
- non-posted DSB writes actually get blocked by the anti-collision
  logic, but unfortunately this ends up hogging the bus for
  long enough that unrelated parallel CPU MMIO accesses start
  to disappear instead

Even though we are updating the LUT during vblank we aren't
immune to the anti-collision logic because it kicks in briefly
for pipe prefill (initiated at frame start). The safe time
window for performing the LUT update is thus between the
undelayed vblank and frame start. Turns out that with low
enough CDCLK frequency (DSB execution speed depends on CDCLK)
we can exceed that.

As we are currently using non-posted writes for the legacy LUT
updates, in which case we can hit the far more severe failure
mode. The problem is exacerbated by the fact that non-posted
writes are much slower than posted writes (~4x it seems).

To mititage the problem let's switch to using posted DSB
writes for legacy LUT updates (which will involve using the
double write approach to avoid other problems with DSB
vs. legacy LUT writes). Despite writing each register twice
this will in fact make the legacy LUT update faster when
compared to the non-posted write approach, making the
problem less likely to appear. The failure mode is also
less severe.

This isn't the 100% solution we need though. That will involve
estimating how long the LUT update will take, and pushing
frame start and/or delayed vblank forward to guarantee that
the update will have finished by the time the pipe prefill
starts...

Cc: stable@vger.kernel.org
Fixes: 34d8311f4a1c ("drm/i915/dsb: Re-instate DSB for LUT updates")
Fixes: 25ea3411bd23 ("drm/i915/dsb: Use non-posted register writes for legacy LUT")
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12494
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241120164123.12706-3-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
(cherry picked from commit 2504a316b35d49522f39cf0dc01830d7c36a9be4)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>