ARM: dts: msm: optimize time of backlight up on spyro wdp

Set backlight ELVSS power rail voltage of panel to -2.2v,
enable backlight control in order to optimize time of backlight up.

Change-Id: I2d8364b7d0d78f0c1ce8365b8f379d728760563b
Signed-off-by: Xipeng Gu <guxipeng@codeaurora.org>