drm/vmwgfx: Fix large topology crash

The previous attempt at this had an issue with with num_clips > 1
because it would always end up using the coordinates of the last
clip while using width and height calculated from the bounding
box of all the clips.

So if the last clip happens to be not at the top-left corner of
the bounding box, the CPU blit operation would go out of bounds.

The original intent was to coalesce all the clips into one blit,
and to do that we need to also track the starting point of the
content buffer.

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