drm/amd/display: Add NULL pointer check for kzalloc

[Why & How]
Check return pointer of kzalloc before using it.
Reviewed-by: Alex Hung <alex.hung@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@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/resource/dcn401/dcn401_resource.c

@@ -1273,6 +1273,8 @@ static struct hpo_dp_link_encoder *dcn401_hpo_dp_link_encoder_create(
 
 	/* allocate HPO link encoder */
 	hpo_dp_enc31 = kzalloc(sizeof(struct dcn31_hpo_dp_link_encoder), GFP_KERNEL);
+	if (!hpo_dp_enc31)
+		return NULL; /* out of memory */
 
 #undef REG_STRUCT
 #define REG_STRUCT hpo_dp_link_enc_regs