drm/amd/display: Minor code cleanup

[WHY]
There is an extra null check for fs_params in the
build_freesync_hdr function detected by Smatch.

[HOW]
1) Remove the extra null check since it is checked in the
caller.

Signed-off-by: SivapiriyanKumarasamy <sivapiriyan.kumarasamy@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>