MdeModulePkg/PciBus: Restrict one VGA per HostBridge not Segment

MdeModulePkg/PciBus: Restrict one VGA per HostBridge not Segment

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1109

Today's restriction of VGA device is to have only one VGA device
enabled per PCI segment. It's not correct because several segments
may share one IO / MMIO address space.
We should restrict to have one VGA per Host Bridge because each
Host Bridge has its only IO / MMIO address space.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>