tests/gen7_forcewake_mt: use local mmio variable

igfx_get_mmio() uses the global mmio variable by accident. Use a local
variable instead.

The intention is to rename the global variable later on, so shadowing it
here does not matter.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>

tests/gen7_forcewake_mt.c

@@ -88,6 +88,7 @@ static struct pci_device *__igfx_get(void)
 static void *igfx_get_mmio(void)
 {
 	struct pci_device *pci = __igfx_get();
+	void *mmio = NULL;
 	int error;
 
 	igt_skip_on(pci == NULL);