drm/amdgpu: save/restore backlight level in legacy dce code

Save/restore the backlight level scratch register in S3/S4 so the
backlight level comes back at the previously requested level.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=199047
Fixes: 4ec6ecf48c64d (drm/amdgpu: drop scratch regs save and restore from S3/S4 handling)
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/amd/amdgpu/atombios_encoders.h b/drivers/gpu/drm/amd/amdgpu/atombios_encoders.h
index 2bdec40..f77cbde 100644
--- a/drivers/gpu/drm/amd/amdgpu/atombios_encoders.h
+++ b/drivers/gpu/drm/amd/amdgpu/atombios_encoders.h
@@ -25,6 +25,11 @@
 #define __ATOMBIOS_ENCODER_H__
 
 u8
+amdgpu_atombios_encoder_get_backlight_level_from_reg(struct amdgpu_device *adev);
+void
+amdgpu_atombios_encoder_set_backlight_level_to_reg(struct amdgpu_device *adev,
+						   u8 backlight_level);
+u8
 amdgpu_atombios_encoder_get_backlight_level(struct amdgpu_encoder *amdgpu_encoder);
 void
 amdgpu_atombios_encoder_set_backlight_level(struct amdgpu_encoder *amdgpu_encoder,