media: i2c: st-vgxy61: Fix regulator counter underflow

Previously regulators were enabled on probe and never again.
However, as regulators are disabled on power off. After a second power off
the regulator counter will underflow. Plus regulators are not required
for probing the sensor, but for streaming.
Fix this by enabling regulators on power on to balance regulator counter
properly.

Signed-off-by: Benjamin Mugnier <benjamin.mugnier@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>