drm/amdgpu: Restore LCD backlight level on resume

Instead of only enabling the backlight (which seems to set it to max
brightness), just re-set the current backlight level, which also takes
care of enabling the backlight if necessary.

Port of radeon commit:
drm/radeon: Restore LCD backlight level on resume (>= R5xx)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/amdgpu/atombios_encoders.c

@@ -1279,8 +1279,7 @@ amdgpu_atombios_encoder_setup_dig(struct drm_encoder *encoder, int action)
 			amdgpu_atombios_encoder_setup_dig_encoder(encoder, ATOM_ENCODER_CMD_DP_VIDEO_ON, 0);
 		}
 		if (amdgpu_encoder->devices & (ATOM_DEVICE_LCD_SUPPORT))
-			amdgpu_atombios_encoder_setup_dig_transmitter(encoder,
-							       ATOM_TRANSMITTER_ACTION_LCD_BLON, 0, 0);
+			amdgpu_atombios_encoder_set_backlight_level(amdgpu_encoder, dig->backlight_level);
 		if (ext_encoder)
 			amdgpu_atombios_encoder_setup_external_encoder(encoder, ext_encoder, ATOM_ENABLE);
 	} else {