mdp3: Add battery FET regulator usage

During idle power-collapse using smart-panel, when PMIC chip goes
to sleep, it puts battery FET (batfet) to low power mode (LPM)
which allows less than 100mA current to be drawn. Since
smart-panel is in self-refresh mode, it can still draw more than
100mA current causing PMIC and MSM reset. To fix this issue,
batfet has to be kept in high power mode (HPM). In order to keep
batfet in HPM, display driver enables batfet regulator whenever
display panel is on.

Change-Id: Ibc41ec2db6ba5b8510a02f3950d7453aa2803606
Signed-off-by: Terence Hampson <thampson@codeaurora.org>