msm: mdss: dsi: add pinctrl support

Add pinctrl support for the installation
of the required GPIOs from DT. Document
properties in bindings.
Make appropriate changes in the driver to
initialize and set the pins to active and sleep
states during unblank and blank stages respectively
in DSI driver.

Change-Id: Ib58d9162d915adace38ca36a0a3af9fa964d8039
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>