msm: camera: isp: acquire tasklet cmd before processing top half

If enqueue_tasklet fails, the payload is leaked because the release
happens in bottom_half which will never be called. Acquire tasklet
cmd before processing IRQ so that no resource is allocated unless
IRQ can be scheduled to tasklet.

Change-Id: I7c38189055c50300ca643a8e94f16a0598b435ee
Signed-off-by: Junzhe Zou <jnzhezou@codeaurora.org>