MdeModulePkg/BootGraphicsResourceTableDxe: don't allocate below 4 GB

The BGRT table has an 8 byte field for the memory address of the image
data, and yet the driver explicitly allocates below 4 GB. This results
in an ASSERT() on systems that do not have any memory below 4 GB to begin
with.

Since neither the PI, the UEFI or the ACPI spec contain any mention of
why this data should reside below 4 GB, replace the allocation call
with an ordinary AllocatePages() call.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>