drm/i915/gvt: Fix incorrect PCI BARs reporting

Looking at our virtual PCI device, we can see surprising Region 4 and Region 5.
00:10.0 VGA compatible controller: Intel Corporation Sky Lake Integrated Graphics (rev 06) (prog-if 00 [VGA controller])
        ....
        Region 0: Memory at 140000000 (64-bit, non-prefetchable) [size=16M]
        Region 2: Memory at 180000000 (64-bit, prefetchable) [size=1G]
        Region 4: Memory at <ignored> (32-bit, non-prefetchable)
        Region 5: Memory at <ignored> (32-bit, non-prefetchable)
        Expansion ROM at febd6000 [disabled] [size=2K]

The fact is that we only implemented BAR0 and BAR2. Surprising Region 4 and
Region 5 are shown because we report their size as 0xffffffff. They should
report size 0 instead.

BTW, the physical GPU has a PIO BAR. GVTg hasn't implemented PIO access, so
we ignored this BAR for vGPU device.

v2: fix BAR size value calculation.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1458032
Signed-off-by: Changbin Du <changbin.du@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>