DisplayPort kernel driver passes error code/status to hardware composer
via the dp_hotplug_error_code sysfs file. When HWC receives/consumes
the error, it will write "0" into that file to reset the error code. So
this file must be readable and writable by the HWC code running with
"system" user/group permissions.
Previously we used to set the ownership to system:system, but in order
to be more consistent with the rest of sysfs files in that directory,
we can use root:graphics instead with permissions 0664. HWC runs
under the "graphics" group, so this should allow HWC write access.
Bug: 324953626
Test: checked permissions of dp_hotplug_error_code sysfs
Change-Id: Idf65acc12d158a78565c41f4e2aea24362e2cdff
b5769ac1f1