drm/amd/display: Test panel power savings sysfs

drm/amd/display: Test panel power savings sysfs

[WHAT]
Add KUnit coverage using a DRM-managed connector fixture.

Tests cover show() mapping immediate-disable to 0 and reporting an active
ABM level, and store() handling the disable mapping, the forbidden update,
invalid text, and out-of-range input. show() writes through a page-backed
buffer because sysfs_emit() requires one.

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>