drm/i915: Fix scanoutpos calculations for interlaced modes

The scanline counter counts lines in the current field, not the entire
frame. But the crtc_ timings are the values for the entire frame. Divide
the vertical timings by 2 to make them match the scanline counter.

The rounding was carefully chosen to make it do the right thing wrt. the
observed scanline counter and ISR vblank bit behaviour.

Reviewed-by: mario.kleiner.de@gmail.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -698,6 +698,12 @@ static int i915_get_crtc_scanoutpos(struct drm_device *dev, int pipe,
 	vbl_start = mode->crtc_vblank_start;
 	vbl_end = mode->crtc_vblank_end;
 
+	if (mode->flags & DRM_MODE_FLAG_INTERLACE) {
+		vbl_start = DIV_ROUND_UP(vbl_start, 2);
+		vbl_end /= 2;
+		vtotal /= 2;
+	}
+
 	ret |= DRM_SCANOUTPOS_VALID | DRM_SCANOUTPOS_ACCURATE;
 
 	/*