drm/msm/sde: take irq callback lock before reading cb list

Take the callback spinlock before checking whether the callback
table list is empty. This resolves a race condition where the
callback list could be empty during the servicing of an
interrupt.

Change-Id: I8d59c0211526173ce98c8ca2dac36ec4743dc8f8
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>