OvmfPkg: non-null PcdLib instance for the CSM VideoDxe

OvmfPkg: non-null PcdLib instance for the CSM VideoDxe

VideoDxe is a UEFI_DRIVER, so it has by default a null instance
of PcdLib.  It accesses two PCDs that are now dynamic
(gEfiMdeModulePkgTokenSpaceGuid.PcdVideoHorizontalResolution
and gEfiMdeModulePkgTokenSpaceGuid.PcdVideoVerticalResolution).
Similar to r15362 (OvmfPkg: non-null PcdLib instance for
GraphicsConsoleDxe, 2014-03-22), we need to specify a non-null
instance of PcdLib.

This patch unbreaks the CSM VideoDxe module for OvmfPkg.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15421 6f19259b-4bc3-4df7-8a09-765794883524