drm/vmwgfx: Fixed topology boundary checking for Screen Targets

For a Screen Target capable display device, the display topology is
limited by SVGA_REG_MAX_PRIMARY_BOUNDING_BOX_MEM.  Two values are
checked against this limit:
  1.  Size of the bounding box enclosing all the displays, and
  2.  Size of the total number of displays, e.g. framebuffers

The limitations above mean we do not have exact max width and
height for the topology.  The best current option is to set those to
the maximum texture width/height.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>