drm/amd/display: add HDR multiplier support

With `dc_fixpt_from_s3132()` translation, we can just use it to set
hdr_mult.

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