drm/amd/display: Test backlight level programming path

drm/amd/display: Test backlight level programming path

[WHAT]
Add KUnit coverage for the backlight level write path in
amdgpu_dm_backlight.c.

Tests cover stream lookup hit/miss, the connector-off and no-stream early
returns, and the AUX/PWM/idle-reallow programming branches. The DC
programming calls stay safe by injecting a NULL power module and keeping
idle optimizations a no-op via disable_idle_power_optimizations.

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>