drm/amd/display: Force PWM backlight on Lenovo Legion 5 15ARH05

The Lenovo Legion 5 15ARH05 (Renoir) ships a BOE 0x08DF eDP panel that
advertises AUX/DPCD backlight control, so amdgpu's automatic detection
(amdgpu_backlight == -1) selects AUX. On this panel the AUX backlight
path has no effect: brightness writes are accepted but the panel level
never changes, the display is stuck at a fixed brightness and
max_brightness is reported as a bogus 511000. As a result neither the
desktop brightness slider nor the brightness hotkeys do anything.

Forcing PWM backlight (amdgpu.backlight=0) restores working control:
max_brightness becomes 65535 and the level tracks writes. This has long
been applied by users as a manual kernel-parameter workaround.

Extend the generic panel backlight quirk with a force_pwm flag, add an
entry for the Legion 5 15ARH05 / BOE 0x08DF panel, and have amdgpu
disable AUX backlight (use PWM) when the quirk matches and the user
lets the driver auto-select the backlight type.

Signed-off-by: Alessandro Rinaldi <ale@alerinaldi.it>
Tested-by: Alessandro Rinaldi <ale@alerinaldi.it>
Reviewed-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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