drm/amd/display: Check denominator pbn_div before used

[WHAT & HOW]
A denominator cannot be 0, and is checked before used.

This fixes 1 DIVIDE_BY_ZERO issue reported by Coverity.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -7561,7 +7561,7 @@ static int dm_update_mst_vcpi_slots_for_dsc(struct drm_atomic_state *state,
 			}
 		}
 
-		if (j == dc_state->stream_count)
+		if (j == dc_state->stream_count || pbn_div == 0)
 			continue;
 
 		slot_num = DIV_ROUND_UP(pbn, pbn_div);