msm: camera: vreg: Fix vreg power down issue

We see that some of the voltages used are not
turned off after use. This is happening because
of incorrect usage of vreg parameters.

In cases when userspace sensor driver requests
for a specific voltage regulator and if same is
not present in DTSI file, we should not touch
that regulator. This change ensures that.

Change-Id: Ie263d6b2ac80b7c8d26a7236e40fdd5526b6a599
Signed-off-by: Vijay kumar Tumati <vtumati@codeaurora.org>