msm: camera: Change sensor power down sequence

Call stop stream at the beginning of power down to make sure
sensor stops at frame boundary. This is to avoid spurious signals
from being picked up by receiver.

Change-Id: I61eed60fce38cc121312c3c2687cc96587525761
Signed-off-by: Sreesudhan Ramakrish Ramkumar <srramku@codeaurora.org>