IntelSiliconPkg IntelVTdDxe: Use ACPI table event to get DMAR table

IntelSiliconPkg IntelVTdDxe: Use ACPI table event to get DMAR table

Use ACPI table event to get DMAR table instead of using ACPI SDT
notification as ACPI SDT is optional and the default value of
PcdInstallAcpiSdtProtocol is FALSE in MdeModulePkg.dec.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>