regulator: qpnp-labibb-regulator: support configuring LAB CURRENT_SENSE

As per the hardware documentation, current sense gain for LAB regulator
should be configured to 1.5 times for AMOLED mode. Add
support for that.

While at it, add support for configuring current sense gain through a
device tree property "qcom,qpnp-lab-current-sense". If the property is
specified, then allow overriding the current sense gain irrespective of
the panel type.

Change-Id: I7567eedb8331c70de715f2b392cbb6e560ec4622
Signed-off-by: Ravindranath Thiyagarajan <rthiyaga@codeaurora.org>