drm/amd/display: add NULL checks for clock manager pointer

This fixes kernel NULL pointer dereferences on shutdown:
RIP: 0010:build_audio_output.isra.0+0x97/0x110 [amdgpu]
RIP: 0010:enable_link_dp+0x186/0x300 [amdgpu]
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Ahzo <Ahzo@tutanota.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -1529,7 +1529,7 @@ static enum dc_status enable_link_dp(
 
 	pipe_ctx->stream_res.pix_clk_params.requested_sym_clk =
 			link_settings.link_rate * LINK_RATE_REF_FREQ_IN_KHZ;
-	if (!apply_seamless_boot_optimization)
+	if (state->clk_mgr && !apply_seamless_boot_optimization)
 		state->clk_mgr->funcs->update_clocks(state->clk_mgr, state, false);
 
 	dp_enable_link_phy(

drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c

@@ -1161,8 +1161,9 @@ static void build_audio_output(
 		}
 	}
 
-	if (pipe_ctx->stream->signal == SIGNAL_TYPE_DISPLAY_PORT ||
-			pipe_ctx->stream->signal == SIGNAL_TYPE_DISPLAY_PORT_MST) {
+	if (state->clk_mgr &&
+		(pipe_ctx->stream->signal == SIGNAL_TYPE_DISPLAY_PORT ||
+			pipe_ctx->stream->signal == SIGNAL_TYPE_DISPLAY_PORT_MST)) {
 		audio_output->pll_info.dp_dto_source_clock_in_khz =
 				state->clk_mgr->funcs->get_dp_ref_clk_frequency(
 						state->clk_mgr);