drm/i915/gvt: cleanup opregion memory allocation code

According to the spec, ACPI OpRegion must be placed at a physical address
below 4G. That is, for a vGPU it must be associated with a GPA below 4G,
but on host side, it doesn't matter where the backing pages actually are.
So when allocating pages from host, the GFP_DMA32 flag is unnecessary.

Also the allocation is from a sleepable context, so GFP_ATOMIC is also
unnecessary.

This patch also removes INTEL_GVT_OPREGION_PORDER and use get_order()
instead.

Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>