OvmfPkg/PlatformInitLib: Set dynamic MMIO window size to 1/4

Large multi-GPU passthrough configurations can exhaust the current
MMIO window sizing rule (1/8 of guest physical address space),
especially with newer GPUs that have very large BAR requirements and
alignment overhead.

With 46-bit guest physical addressing, the 1/8 rule provides an 8TB
MMIO window. Newer GPUs with 512GB VRAM can exhaust an 8TB window
when passing through up to 8 devices.

Increase the MMIO window size to 1/4 of guest physical address space.
This provides a 16TB MMIO window on a 46-bit setup.

Tested with 8-GPU passthrough (512GB VRAM per GPU). With this
change, all GPUs are visible and functional in the VM; without it,
one GPU was unusable.

Signed-off-by: ramkumar.s <ramkumar.s@nutanix.com>