drm/i915/dsb: Fix DSB vblank waits when using VRR

Looks like the undelayed vblank gets signalled exactly when
the active period ends. That is a problem for DSB+VRR when
we are already in vblank and expect DSB to start executing
as soon as we send the push. Instead of starting, the DSB
just keeps on waiting for the undelayed vblank which won't
signal until the end of the next frame's active period,
which is far too late.

The end result is that DSB won't have even started
executing by the time the flips/etc. have completed.
We then wait for an extra 1ms, after which we terminate
the DSB and report a timeout:
[drm] *ERROR* [CRTC:80:pipe A] DSB 0 timed out waiting for idle (current head=0xfedf4000, head=0x0, tail=0x1080)

To fix this let's configure DSB to use the so called VRR
"safe window" instead of the undelayed vblank to trigger
the DSB vblank logic, when VRR is enabled.

Cc: stable@vger.kernel.org
Fixes: 34d8311f ("drm/i915/dsb: Re-instate DSB for LUT updates")
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9927


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240306040806.21697-3-ville.syrjala@linux.intel.com


Reviewed-by: Animesh Manna <animesh.manna@intel.com>