drm/amd/display: OLED panel backlight adjust not work with external display connected

[Why]
amdgpu_dm->backlight_caps is for single eDP only. the caps are upddated
for very connector. Real eDP caps will be overwritten by other external
display. For OLED panel, caps->aux_support is set to 1 for OLED pnael.
after external connected, caps+.aux_support is set to 0. This causes
OLED backlight adjustment not work.

[How]
within update_conector_ext_caps, backlight caps will be updated only for
eDP connector.

Cc: [e-mail]
Signed-off-by: hersen wu <[e-mail]>
Reviewed-by: Nicholas Kazlauskas <[e-mail]>
Acked-by: Rodrigo Siqueira <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 	struct amdgpu_display_manager *dm;
 	struct drm_connector *conn_base;
 	struct amdgpu_device *adev;
+	struct dc_link *link = NULL;
 	static const u8 pre_computed_values[] = {
 		50, 51, 52, 53, 55, 56, 57, 58, 59, 61, 62, 63, 65, 66, 68, 69,
 		71, 72, 74, 75, 77, 79, 81, 82, 84, 86, 88, 90, 92, 94, 96, 98};
 
 	if (!aconnector || !aconnector->dc_link)
+		return;
+
+	link = aconnector->dc_link;
+	if (link->connector_signal != SIGNAL_TYPE_EDP)
 		return;
 
 	conn_base = &aconnector->base;