drm/vblank: Use spin_(un)lock_irq() in drm_legacy_vblank_post_modeset()

This function is only ever called from ioctl context, so we're
guaranteed to have interrupts enabled. Stop using the irqsave/irqrestore
variants of spin_(un)lock_irq() to make this more obvious.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200720190736.180297-4-lyude@redhat.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_vblank.c

@@ -1551,7 +1551,6 @@ static void drm_legacy_vblank_post_modeset(struct drm_device *dev,
 					   unsigned int pipe)
 {
 	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
-	unsigned long irqflags;
 
 	/* vblank is not initialized (IRQ not installed ?), or has been freed */
 	if (!drm_dev_has_vblank(dev))
@@ -1561,9 +1560,9 @@ static void drm_legacy_vblank_post_modeset(struct drm_device *dev,
 		return;
 
 	if (vblank->inmodeset) {
-		spin_lock_irqsave(&dev->vbl_lock, irqflags);
+		spin_lock_irq(&dev->vbl_lock);
 		drm_reset_vblank_timestamp(dev, pipe);
-		spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
+		spin_unlock_irq(&dev->vbl_lock);
 
 		if (vblank->inmodeset & 0x2)
 			drm_vblank_put(dev, pipe);