drm/msm/sde: fix IRQ callback list adding issue

Remove the interrupt callback list initalization from interrupt
function, and put it into the event enablement function.

Use case is interrupt enabled through IOCTL from userspace, and
device resumed from IPC. Then the interrupt function is called
twice, and the same IRQ callback is registered twice. To initialize
the IRQ callback list for the second time would cause disconnection
for the callback node, while the node is still in the callback
table list. Error would happen when attempt to add the node into
table list again.

Change-Id: I0c441b50354ae6f1a54b1aebfbe7063ffe535d43
Signed-off-by: Xu Yang <yangxu@codeaurora.org>