drm/i915/vrr: Make registers latch in a consitent place on icl/tgl

Account for the framestart delay when calculating the "pipeline full"
value for icl/tgl vrr. This puts the start of vblank (ie. where the
double bufferd registers get latched) to a consistent place regardless
of what framestart delay value is used. framestart delay does not
change where start of vblank occurs in non-vrr mode and I can't see
any reason why we'd want different behaviour in vrr mode.

Currently framestart delay is always set to 1, and the hardcoded 4
scanlines in the code means we're currently delaying the start of
vblank by three extra lines. And with framestart delay set to 4 we'd
have no extra delay.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221202134412.21943-2-ville.syrjala@linux.intel.comReviewed-by: Manasi Navare <manasi.d.navare@intel.com>

drivers/gpu/drm/i915/display/intel_vrr.c

@@ -153,18 +153,9 @@ intel_vrr_compute_config(struct intel_crtc_state *crtc_state,
 		crtc_state->vrr.guardband =
 			crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay;
 	} else {
-		/*
-		 * FIXME: s/4/framestart_delay/ to get consistent
-		 * earliest/latest points for register latching regardless
-		 * of the framestart_delay used?
-		 *
-		 * FIXME: this really needs the extra scanline to provide consistent
-		 * behaviour for all framestart_delay values. Otherwise with
-		 * framestart_delay==4 we will end up extending the min vblank by
-		 * one extra line.
-		 */
 		crtc_state->vrr.pipeline_full =
-			min(255, crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay - 4 - 1);
+			min(255, crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay -
+			    crtc_state->framestart_delay - 1);
 	}
 
 	crtc_state->mode_flags |= I915_MODE_FLAG_VRR;