msm: mdss: initialize external BL level when fb probe

Initialize external backlight level to -1 when fb probe.
This is to make sure the external backlight level is
invalid the first time overlay kickoff.

CRs-Fixed: 2021142

Change-Id: I973b184903cdb5b378326b23ca5c958e4b868c76
Signed-off-by: Xu Yang <yangxu@codeaurora.org>