drm/amd/display: Skip wbscl_set_scaler_filter if filter is null

[ Upstream commit c4d31653c03b90e51515b1380115d1aedad925dd ]

Callers can pass null in filter (i.e. from returned from the function
wbscl_get_filter_coeffs_16p) and a null check is added to ensure that is
not the case.

This fixes 4 NULL_RETURNS issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>