drm/msm/sde: Correct vblank ref count in case of failures

If register IRQ fails, correct vblank ref counts so that
IRQ registration can be tried again.

Change-Id: I4bc0864f182da6ef1a8f1ab5e36892d8201b1195
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>