drm/amd/display: Ensure curve to hw translation succeed

[WHAT & HOW]
Check cm3_helper_translate_curve_to_hw_format runs successfully so the
regamma_params is valid and can be used.

Also revmoed two result assignments.

This fixes 2 CHECKED_RETURN UNUSED_VALUE issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>