drm/amd/display: Change disable backlight ramp change threshold from 0 to maximum value.

Instead of user set brightness with range of percentage,
HLK test set brightness level with range of normal, this will result in
HLK test case set brightness from 0 to 255, DC set brightness with ramp is 0,
and disabled ramp change which will fail the HLK test.
Fix:
In case of unblank stream and turn on edp, change brightness level in
stream to 0xFFFFFFFF(actural maximum level is 0xFF), use that value as
a flag to recogonize this the case of resume from S3.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>