drm/amd/display: Check null pointers before using dc->clk_mgr

[WHY & HOW]
dc->clk_mgr is null checked previously in the same function, indicating
it might be null.

Passing "dc" to "dc->hwss.apply_idle_power_optimizations", which
dereferences null "dc->clk_mgr". (The function pointer resolves to
"dcn35_apply_idle_power_optimizations".)

This fixes 1 FORWARD_NULL issue reported by Coverity.
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@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.c

@@ -5425,7 +5425,8 @@ void dc_allow_idle_optimizations_internal(struct dc *dc, bool allow, char const
 	if (allow == dc->idle_optimizations_allowed)
 		return;
 
-	if (dc->hwss.apply_idle_power_optimizations && dc->hwss.apply_idle_power_optimizations(dc, allow))
+	if (dc->hwss.apply_idle_power_optimizations && dc->clk_mgr != NULL &&
+	    dc->hwss.apply_idle_power_optimizations(dc, allow))
 		dc->idle_optimizations_allowed = allow;
 }