gallium/vbuf: fix buffer reference bugs

gallium/vbuf: fix buffer reference bugs

In two places we called pipe_resource_reference() to remove a reference
to a vertex buffer resource.  But we neglected to check if the buffer was
a user buffer and not a pipe_resource.  This caused us to pass an invalid
pipe_resource pointer to pipe_resource_reference().

Instead of calling pipe_resource_reference(&vbuf->resource, NULL), use
pipe_vertex_buffer_unreference(&vbuf) which checks the is_user_buffer
field and does the right thing.

Also, explicity set the is_user_buffer field to false after setting the
vbuf->resource pointer to out_buffer.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102377
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Bruce Cherniak <bruce.cherniak@intel.com>