drm/amd/display: Fix OLED brightness control on eDP

[Why]
After commit ("drm/amdgpu/display: add support for multiple backlights")
number of eDPs is defined while registering backlight device.
However the panel's extended caps get updated once before register call.
That leads to regression with extended caps like oled brightness control.

[How]
Update connector ext caps after register_backlight_device

Fixes: 7fd13bae

 ("drm/amdgpu/display: add support for multiple backlights")
Link: https://www.reddit.com/r/AMDLaptops/comments/qst0fm/after_updating_to_linux_515_my_brightness/

Signed-off-by: Roman Li <Roman.Li@amd.com>
Tested-by: Samuel Čavoj <samuel@cavoj.net>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jasdeep Dhillon <Jasdeep.Dhillon@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>