disp: msm: sde: add cb func check before enabling irq

Acquiring mutex lock after enabling irq might lead to
deadlock if irq is dispatched immediately in some cases.
This change moves callback func check for irq prior to
enabling the irq line.

Change-Id: Ie1622e1ade268e41e97bf6d7683fa0bd972bcb7c
Signed-off-by: Yashwanth <yvulapu@codeaurora.org>