drm/amd/display: Atomic freesync ASSERT fix

Changes to atomic set property for freesync.

Now In set property, just set the freesync variables and return 0.
Based on the variables call mod_freesync_set_user_enable() inside
commit_tail

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>