msm_fb: display: replace task_tgid_nr with current->pid

rotator session is not be shared by threads with same parent.
Therefore repalce task_tgid_nr(current) with current->pid.

Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
diff --git a/drivers/char/msm_rotator.c b/drivers/char/msm_rotator.c
index 47d7a28..360b71f 100644
--- a/drivers/char/msm_rotator.c
+++ b/drivers/char/msm_rotator.c
@@ -1165,7 +1165,7 @@
 	if (i == MAX_SESSIONS)
 		return -EBUSY;
 
-	filp->private_data = (void *)task_tgid_nr(current);
+	filp->private_data = (void *)current->pid;
 
 	return 0;
 }