drm/amd/display: check TG is non-null before checking if enabled

[Why & How]
If there is no TG allocation we can dereference a NULL pointer when
checking if the TG is enabled.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Taimur Hassan <syed.hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Taimur Hassan

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c

@@ -3278,7 +3278,8 @@ void dcn10_wait_for_mpcc_disconnect(
 		if (pipe_ctx->stream_res.opp->mpcc_disconnect_pending[mpcc_inst]) {
 			struct hubp *hubp = get_hubp_by_inst(res_pool, mpcc_inst);
 
-			if (pipe_ctx->stream_res.tg->funcs->is_tg_enabled(pipe_ctx->stream_res.tg))
+			if (pipe_ctx->stream_res.tg &&
+				pipe_ctx->stream_res.tg->funcs->is_tg_enabled(pipe_ctx->stream_res.tg))
 				res_pool->mpc->funcs->wait_for_idle(res_pool->mpc, mpcc_inst);
 			pipe_ctx->stream_res.opp->mpcc_disconnect_pending[mpcc_inst] = false;
 			hubp->funcs->set_blank(hubp, true);