regulator: qpnp-lcdb: Add a DT property for LDO power-down delay

Add "qcom,pwrdn-delay-ms" device tree property to meet the power
down delay spec for few display panels. Also, enable force-pull
down on module disable to do a quick discharge of the LDO.

Change-Id: I07d55cdbfe3b253579ca3722fc02604a4a7483e0
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>