drm/amd/display: Fix a debugfs null pointer error

[WHY & HOW]
Check whether get_subvp_en() callback exists before calling it.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Alex Hung <alex.hung@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@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_debugfs.c

@@ -3647,12 +3647,16 @@ static int capabilities_show(struct seq_file *m, void *unused)
 	bool mall_supported = dc->caps.mall_size_total;
 	bool subvp_supported = dc->caps.subvp_fw_processing_delay_us;
 	unsigned int mall_in_use = false;
-	unsigned int subvp_in_use = dc->cap_funcs.get_subvp_en(dc, dc->current_state);
+	unsigned int subvp_in_use = false;
+
 	struct hubbub *hubbub = dc->res_pool->hubbub;
 
 	if (hubbub->funcs->get_mall_en)
 		hubbub->funcs->get_mall_en(hubbub, &mall_in_use);
 
+	if (dc->cap_funcs.get_subvp_en)
+		subvp_in_use = dc->cap_funcs.get_subvp_en(dc, dc->current_state);
+
 	seq_printf(m, "mall supported: %s, enabled: %s\n",
 			   mall_supported ? "yes" : "no", mall_in_use ? "yes" : "no");
 	seq_printf(m, "sub-viewport supported: %s, enabled: %s\n",