MdeModulePkg/ConPlatform: Support GOP created as PCI's grandson

The original logic assumes GOP hande is son of PCI handle but it
is not always true.
Below wordings are from UEFI Spec:
If a graphics device supports multiple frame buffers, then
handles for the frame buffers must be created first, and then the
handles for the video output devices can be created as children of
the frame buffer handles.

So the GOP handle could be grandson of the PCI handle.

EfiBootManagerGetGopDevicePath(VideoController) is used to fix
this bug.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>