drm/amd/display: Test backlight ABM property attach

drm/amd/display: Test backlight ABM property attach

[WHAT]
Cover the eDP success path of amdgpu_dm_setup_backlight_device() where the
ABM level property is attached to the connector. A DRM-managed connector
fixture (drmm_mode_config_init + drmm_connector_init) lets the test verify
that one property is added and that its value is ABM_SYSFS_CONTROL.

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>