drm/amd/display: fix dereference of pointer fs_params before it is null checked

Currently there are several instances of pointer fs_params being
dereferenced before fs_params is being null checked.  Fix this by
only dereferencing fs_params after the null check.

Detected by CoverityScan, CID#1475565 ("Dereference before null check")

Fixes: e1e8a020

 ("drm/amd/display: Add support for Freesync 2 HDR and Content to Display Mapping")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>