drm/amd/display: set sdr_ref_white_level to 80 for out_transfer_func

Otherwise this is just initialized to 0. This needs to actually have a
value so that compute_curve can work for PQ EOTF.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Joshua Ashton <joshua@froggi.es>
Co-developed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>