msm: camera: isp: improve flush logic to prevent race condition

Race condition can happen if flush is called when there is a request
applied and waits for regupd. Add the request to wait list after apply
to track it correctly. Also remove flush state to let context to get
notified by regupd after flush is called.

Change-Id: Ib9c5a4b9e7a103d05fb023b442df76d4d3f30cac
Signed-off-by: Junzhe Zou <jnzhezou@codeaurora.org>