Label sysfs node power_mode as sysfs_display.

Label power_mode to sysfs_panel to let it be allowed in dumpstate.

avc log:
08-26 13:07:52.112 12585 12585 W dump_second_dis: type=1400 audit(0.0:22): avc:  denied  { read } for  name="power_mode" dev="sysfs" ino=90675 scontext=u:r:dump_second_display:s0 tcontext=u:object_r:sysfs:s0 tclass=file permissive=0

Test: ls -Z, adb bugreport.
Flag: EXEMPT bugfix
Bug: 358505990
Change-Id: If13b6d231ba06bab6432eeb3aefb78f4f7d384a3
Signed-off-by: attis <attis@google.com>