drm/msm/dsi-staging: handoff backlight level for cont splash

When continuous splash screen feature is enabled, the display panel
is left on from the bootloader. As part of the transition to kernel,
the backlight level also needs to be handed off. Otherwise, it is
possible that the kernel can inadvertently set the backlight level
to zero and result in what appears like a black screen during
the transition from splash screen to boot animation. Add this support to
read the current backlight level for WLED backlight type and set it
during handoff. For other backlight types, set it to a default
value.

CRs-Fixed: 2248169
Change-Id: Ic003a1432eefbf8b9a34457440f995bbe29f5c9b
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>