msm_sensor: fix vreg dt parsing logic

Previously, dt parsing code for sensor power up sequence assumes certain
value and order in "qcom,cam-vreg-name".  Such assumption can't be
enforced by dt itself.  It leads to kernel panic when enum value of a vreg
is larger than total number of specified vregs.

New implementation dynamically search and match available vregs, instead of
relying on order and enum assumptions.

Change-Id: Iea519f2e4b4d805fecf6dd699ade2af4a00caa43
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>