drm/amd/display: Read back max backlight value at boot

[Why]
If there is no program explicitly setting the backlight
brightness (for example, during a minimal install of linux), the
hardware defaults to maximum brightness but the backlight_device
defaults to 0 value.  Thus, settings displays the wrong brightness
value.

[How]
When creating the backlight device, set brightness to max
Signed-off-by: NDavid Francis <David.Francis@amd.com>
Reviewed-by: NHarry Wentland <Harry.Wentland@amd.com>
Acked-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

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

@@ -1340,6 +1340,7 @@ amdgpu_dm_register_backlight_device(struct amdgpu_display_manager *dm)
 	struct backlight_properties props = { 0 };
 
 	props.max_brightness = AMDGPU_MAX_BL_LEVEL;
+	props.brightness = AMDGPU_MAX_BL_LEVEL;
 	props.type = BACKLIGHT_RAW;
 
 	snprintf(bl_name, sizeof(bl_name), "amdgpu_bl%d",