drm/msm/sde: mitigate error log in IRQ handler

In certain race condition scenarios, when unregister
callback is triggered while MDP is handling IRQ, the
irq table might be empty. Mitigate error log in cases
where enable count is 0.

Change-Id: I228e45dc1798614b5e2ca8fd12ac7f7835298aed
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>