drm/amd/display: Ensure array index tg_inst won't be -1

[WHY & HOW]
tg_inst will be a negative if timing_generator_count equals 0, which
should be checked before used.

This fixes 2 OVERRUN issues reported by Coverity.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@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/dc/core/dc_resource.c

@@ -3542,7 +3542,7 @@ static bool acquire_otg_master_pipe_for_stream(
 		if (pool->dpps[pipe_idx])
 			pipe_ctx->plane_res.mpcc_inst = pool->dpps[pipe_idx]->inst;
 
-		if (pipe_idx >= pool->timing_generator_count) {
+		if (pipe_idx >= pool->timing_generator_count && pool->timing_generator_count != 0) {
 			int tg_inst = pool->timing_generator_count - 1;
 
 			pipe_ctx->stream_res.tg = pool->timing_generators[tg_inst];