drm/i915: Calculate vblank timestamping constants before enabling vblank.

This is required to properly initialize vblanks on the active crtc.
Without it drm_calc_vbltimestamp_from_scanoutpos can fail with
crtc 0: Noop due to uninitialized mode.
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: NDaniel Stone <daniels@collabora.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -15239,6 +15239,7 @@ static void intel_sanitize_crtc(struct intel_crtc *crtc)
 	/* restore vblank interrupts to correct state */
 	drm_crtc_vblank_reset(&crtc->base);
 	if (crtc->active) {
+		drm_calc_timestamping_constants(&crtc->base, &crtc->base.hwmode);
 		update_scanline_offset(crtc);
 		drm_crtc_vblank_on(&crtc->base);
 	}