drm/amd/display: Use kfree() to free rgb_user in calculate_user_regamma_ramp()

Use kfree() instead of kvfree() to free rgb_user in
calculate_user_regamma_ramp() because the memory is allocated with
kcalloc().

Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>