drm/amd/display: Fix default DC and AC levels

JIRA: https://issues.redhat.com/browse/RHEL-75958

commit 6c56c8ec6f9762c33bd22f31d43af4194d12da53
Author: Mario Limonciello <mario.limonciello@amd.com>
Date:   Wed May 14 16:06:40 2025 -0500

    drm/amd/display: Fix default DC and AC levels

    commit 8b5f3a2 upstream.

    [Why]
    DC and AC levels are advertised in a percentage, not a luminance.

    [How]
    Scale DC and AC levels to supported values.

    Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4221
    Reviewed-by: Alex Hung <alex.hung@amd.com>
    Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
    Signed-off-by: Wayne Lin <wayne.lin@amd.com>
    Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: José Expósito <jexposit@redhat.com>

Author: jexposit

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4849,9 +4849,9 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	amdgpu_acpi_get_backlight_caps(&caps);
 	if (caps.caps_valid && get_brightness_range(&caps, &min, &max)) {
 		if (power_supply_is_system_supplied() > 0)
-			props.brightness = caps.ac_level;
+			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps.ac_level, 100);
 		else
-			props.brightness = caps.dc_level;
+			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps.dc_level, 100);
 		/* min is zero, so max needs to be adjusted */
 		props.max_brightness = max - min;
 		drm_dbg(drm, "Backlight caps: min: %d, max: %d, ac %d, dc %d\n", min, max,