drm/amd/display: Only set default brightness for OLED

[Why]
We used to unconditionally set backlight path as AUX for panels capable
of backlight adjustment via DPCD in set default brightness.

[How]
This should be limited to OLED panel only since we control backlight via
PWM path for SDR mode in LCD HDR panel.

Reviewed-by: Krunoslav Kovac <krunoslav.kovac@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Camille Cho <Camille.Cho@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>