drm/crc: Only open CRC on atomic drivers when the CRTC is active.

Commit e8fa5671 ("drm: crc: Wait for a frame before returning
from open()") adds a wait for CRC frame, but with the CRTC off
this will never be generated. For atomic drivers we know if a CRTC
is active through crtc_state->active, so when inactive reject the
open with -EIO.

Just like with the previous patch changing debugfs opening semantics,
this patch has been tested against igt.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: e8fa5671 ("drm: crc: Wait for a frame before returning from open()")
Testcase: debugfs_test.read_all_entries
Link: http://patchwork.freedesktop.org/patch/msgid/15f9d300-65d3-63aa-00e3-e83f5e4d5a7a@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_debugfs_crc.c

@@ -154,6 +154,19 @@ static int crtc_crc_open(struct inode *inode, struct file *filep)
 	size_t values_cnt;
 	int ret = 0;
 
+	if (drm_drv_uses_atomic_modeset(crtc->dev)) {
+		ret = drm_modeset_lock_interruptible(&crtc->mutex, NULL);
+		if (ret)
+			return ret;
+
+		if (!crtc->state->active)
+			ret = -EIO;
+		drm_modeset_unlock(&crtc->mutex);
+
+		if (ret)
+			return ret;
+	}
+
 	spin_lock_irq(&crc->lock);
 	if (!crc->opened)
 		crc->opened = true;