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: stable@vger.kernel.org
Signed-off-by: hersen wu <hersenxs.wu@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>