drm/amd/display: fix a missing null check on a failed kzalloc

Currently the allocation of config may fail and a null pointer
dereference on config can occur.  Fix this by added a null
check on a failed allocation of config.

Addresses-Coverity: ("Dereference null return")
Fixes: c2cd9d04 ("drm/amd/display: Hook up calls to do stereo mux and dig programming to stereo control interface")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1259,6 +1259,8 @@ bool dc_set_generic_gpio_for_stereo(bool enable,
 	struct gpio_generic_mux_config *config = kzalloc(sizeof(struct gpio_generic_mux_config),
 			   GFP_KERNEL);
 
+	if (!config)
+		return false;
 	pin_info = dal_gpio_get_generic_pin_info(gpio_service, GPIO_ID_GENERIC, 0);
 
 	if (pin_info.mask == 0xFFFFFFFF || pin_info.offset == 0xFFFFFFFF) {