drm/amd/display: camel case cleanup in color_gamma file

Rename mapUserRamp to map_user_ramp and doClamping to do_clamping

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>