drm/i915/display: Wait for scl start instead of dsb_wait_vblanks

drm/i915/display: Wait for scl start instead of dsb_wait_vblanks

Until LNL, intel_dsb_wait_vblanks() used to wait for the undelayed vblank
start. However, from PTL onwards, it waits for the start of the
safe-window defined by the number of lines programmed in the register
TRANS_SET_CONTEXT_LATENCY. This change was introduced to move the SCL
window out of the vblank region, supporting modes with higher refresh
rates and smaller vblanks. This change introduces a "safe window" a
scanline range from (undelayed vblank - SCL) to (delayed vblank - SCL).

As a result, on PTL+ platforms, the DSB wait for vblank completes exactly
SCL lines earlier than the undelayed vblank start (safe window start).
If the flip occurs in the active region and the push happens before the
vmin decision boundary, the DSB wait fires early, and the push is sent
inside this safe window. In such cases, the push bit is cleared at the
delayed vblank, but our wait logic does not account for the early trigger,
leading to DSB poll errors.

To fix this, we add an explicit wait for the end of the safe window i.e.,
the scanline range from (undelayed vblank - SCL) to (delayed vblank - SCL).
Once past this window, we are exactly SCL lines away from the delayed
vblank, and our existing wait logic works as intended.

This additional wait is only effective if the push occurs before the vmin
decision boundary. If the push happens after the boundary, the hardware
already guarantees we're SCL lines away from the delayed vblank, and the
extra wait becomes a no-op.

v2:
- Use helpers for safe window start/end. (Ville)
- Move the extra wait inside the helper to wait for delayed vblank. (Ville)
- Update the commit message.

v3:
- Add more documentation for explanation for the wait. (Ville)
- Rename intel_vrr_vmin_safe_window_start/end as this is vmin safe
  window. (Ville)
- Minor refactoring to align with the code. (Ville)
- Update the commit message for more clarity.

v4:
- Retain name for intel_vrr_safe_window_start as it doesn't change with
  vmin/vmax etc. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250924141542.3122126-7-ankit.k.nautiyal@intel.com