OvmfPkg/OvmfPkg.dec: Add PCD definitions used by copied CSM modules

OvmfPkg/OvmfPkg.dec: Add PCD definitions used by copied CSM modules

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1811

This commit will add the PCD definitions consumed by the duplicated
drivers:

* VideoDxe
* LegacyBiosDxe

into the OvmfPkg DEC file.

Please note that, instead of adding these PCDs under section:
[PcdsFixedAtBuild, PcdsDynamic, PcdsDynamicEx, PcdsPatchableInModule]

as in IntelFrameworkModulePkg.dec file, they are added in section:
[PcdsFixedAtBuild]

in OvmfPkg.dec instead.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>