msm: camera: vreg: Fix vreg power up issue

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. Else,
we may power up default regulator unnecessarily.

Change-Id: I3163ee0ab747899bacd7aab0dac0ac5a6ac5bfa6
Signed-off-by: Vijay kumar Tumati <vtumati@codeaurora.org>