disp: msm: sde: fix potential race condition

Move the hist irq handling out of callback function, i.e., the hw
interrupt irq_lock context, to avoid dead lock between crtc spin_lock
and irq_lock. This change also extends crtc spin_lock coverage in
_sde_cp_crtc_enable_hist_irq to prevent null pointer dereference on
event node, which can be deleted during crtc event de-registration.

Change-Id: Iadaed54ab93c4c4abe065a8762d2addccb0c65c6
Signed-off-by: Ping Li <pingli@codeaurora.org>