svga: fix incompatible bind flags at buffer validation time

svga: fix incompatible bind flags at buffer validation time

At buffer resource validation time, if the resource handle is not yet
created and if the initial buffer bind flags and the tobind flags are
incompatible, just use the tobind flags to create the resource handle.
On the other hand, if the bind flags are compatible, we can combine
the bind flags for the resource handle creation.

Fixes piglit gl-3.1-buffer-bindings crash.

Reviewed-by: Brian Paul <brianp@vmware.com>