target: msm8996: support configurable rearview display

In current code, early rearview is hardcoded to be displayed
on the first inited display. User needs to modify code when
user wants to move rearview to other displays. This change
is to provide one easy way for target rearview display
configuration, allowing target display can be selected
via fastboot command.

Change-Id: Idbca34d49d0e2e75cdff3975169264fe955ec40c
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>