drm/i915: Return zero as the scanline counter for disabled pipes

We print the scanline counters as unsigned integers so the -1
here just makes the debugs/traces look a bit messy. Zero seems
equally valid for this usecase.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210304170421.10901-4-ville.syrjala@linux.intel.comReviewed-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -794,7 +794,7 @@ static int __intel_get_crtc_scanline(struct intel_crtc *crtc)
 	int position, vtotal;
 
 	if (!crtc->active)
-		return -1;
+		return 0;
 
 	vblank = &crtc->base.dev->vblank[drm_crtc_index(&crtc->base)];
 	mode = &vblank->hwmode;