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

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>

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_dwb_scl.c

@@ -690,6 +690,9 @@ static void wbscl_set_scaler_filter(
 	int pair;
 	uint16_t odd_coef, even_coef;
 
+	if (!filter)
+		return;
+
 	for (phase = 0; phase < (NUM_PHASES / 2 + 1); phase++) {
 		for (pair = 0; pair < tap_pairs; pair++) {
 			even_coef = filter[phase * taps + 2 * pair];