drm/amd/display: Fix programming backlight on OLED panels

commit 38077562 ("drm/amd/display: Implement new
backlight_level_params structure") adjusted DC core to require
the backlight type to be programmed in the dc link when changing
brightness.  This isn't initialized in amdgpu_dm for OLED panels
though which broke brightness.

Explicitly initialize when aux support is enabled.

Reported-and-tested-by: Luke Jones <luke@ljones.dev>
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3792


Fixes: 38077562 ("drm/amd/display: Implement new backlight_level_params structure")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://lore.kernel.org/r/20241128032200.2085398-1-superm1@kernel.org


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>