drm/radeon: support backlight control for UNIPHY3

BugLink: http://bugs.launchpad.net/bugs/1615620

commit d3200be6 upstream.

Same interface as other UNIPHY blocks
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -120,6 +120,7 @@ atombios_set_backlight_level(struct radeon_encoder *radeon_encoder, u8 level)
 		case ENCODER_OBJECT_ID_INTERNAL_KLDSCP_LVTMA:
 		case ENCODER_OBJECT_ID_INTERNAL_UNIPHY1:
 		case ENCODER_OBJECT_ID_INTERNAL_UNIPHY2:
+		case ENCODER_OBJECT_ID_INTERNAL_UNIPHY3:
 			if (dig->backlight_level == 0)
 				atombios_dig_transmitter_setup(encoder, ATOM_TRANSMITTER_ACTION_LCD_BLOFF, 0, 0);
 			else {