svga: change error handling convention for svga_set_stream_output()

svga: change error handling convention for svga_set_stream_output()

In general, the functions which emit commands to the command buffer check
for failure and return a PIPE_ERROR_x code.  It's up to the caller to
flush the buffer and retry the command.

But svga_set_stream_output() did its own flushing and the callers never
checked the return value (though, it would always be PIPE_OK) in practice.

This patch changes svga_set_stream_output() so that it does not call
svga_context_flush() when the buffer is full.  And we update the callers
to check the return value as we do for other functions, like
svga_set_shader().

No Piglit regressions.  Also tested w/ Nature demo.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>