q35：Enable TSEG only when G_SMRAME and TSEG_EN both enabled

According to spec:
"TSEG Enable (T_EN): Enabling of SMRAM memory for Extended SMRAM space
only. When G_SMRAME = 1 and TSEG_EN = 1, the TSEG is enabled to appear
in the appropriate physical address space. Note that once D_LCK is set,
this bit becomes read only."

Changed to match the spec description.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Message-Id: <20220615034501.2733802-1-zhenzhong.duan@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/pci-host/q35.c

@@ -379,7 +379,8 @@ static void mch_update_smram(MCHPCIState *mch)
         memory_region_set_enabled(&mch->high_smram, false);
     }
 
-    if (pd->config[MCH_HOST_BRIDGE_ESMRAMC] & MCH_HOST_BRIDGE_ESMRAMC_T_EN) {
+    if ((pd->config[MCH_HOST_BRIDGE_ESMRAMC] & MCH_HOST_BRIDGE_ESMRAMC_T_EN) &&
+        (pd->config[MCH_HOST_BRIDGE_SMRAM] & SMRAM_G_SMRAME)) {
         switch (pd->config[MCH_HOST_BRIDGE_ESMRAMC] &
                 MCH_HOST_BRIDGE_ESMRAMC_TSEG_SZ_MASK) {
         case MCH_HOST_BRIDGE_ESMRAMC_TSEG_SZ_1MB: